Method and apparatus for processing video signal

ABSTRACT

A method for processing a video according to the present invention may comprise: generating a plurality of Most Probable Mode (MPM) candidates; determining whether there is an MPM candidate identical to an intra-prediction mode of a current block among the plurality of MPM candidates; obtaining the intra-prediction mode of the current block, based on a result of the determining; and performing an intra-prediction for the current block, based on the intra-prediction mode of the current block.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Patent Application of PCTInternational Patent Application No. PCT/KR2017/003685 (filed on Apr. 4,2017) under 35 U.S.C. § 371, which claims priority to Korean PatentApplication Nos. 10-2016-0042095 (filed on Apr. 6, 2016) and10-2016-0079635 (filed on Jun. 24, 2016), the teachings of which areincorporated herein in their entireties by reference.

TECHNICAL FIELD

The present invention relates to a method and an apparatus forprocessing video signal.

BACKGROUND ART

Recently, demands for high-resolution and high-quality images such ashigh definition (HD) images and ultra-high definition (UHD) images haveincreased in various application fields. However, higher resolution andquality image data has increasing amounts of data in comparison withconventional image data. Therefore, when transmitting image data byusing a medium such as conventional wired and wireless broadbandnetworks, or when storing image data by using a conventional storagemedium, costs of transmitting and storing increase. In order to solvethese problems occurring with an increase in resolution and quality ofimage data, high-efficiency image encoding/decoding techniques may beutilized.

Image compression technology includes various techniques, including: aninter-prediction technique of predicting a pixel value included in acurrent picture from a previous or subsequent picture of the currentpicture; an intra-prediction technique of predicting a pixel valueincluded in a current picture by using pixel information in the currentpicture; an entropy encoding technique of assigning a short code to avalue with a high appearance frequency and assigning a long code to avalue with a low appearance frequency; etc. Image data may beeffectively compressed by using such image compression technology, andmay be transmitted or stored.

In the meantime, with demands for high-resolution images, demands forstereographic image content, which is a new image service, have alsoincreased. A video compression technique for effectively providingstereographic image content with high resolution and ultra-highresolution is being discussed.

DISCLOSURE Technical Problem

An object of the present invention is intended to provide a method andan apparatus for efficiently performing intra-prediction for anencoding/decoding target block in encoding/decoding a video signal.

An object of the present invention is intended to provide a method andan apparatus for obtaining an intra-prediction mode of anencoding/decoding target block using a neighboring block adjacent to anencoding/decoding target block in encoding/decoding a video signal.

An object of the present invention is intended to provide a method andan apparatus for performing an intra-prediction for an encoding/decodingtarget block based on a plurality of reference lines.

The technical objects to be achieved by the present invention are notlimited to the above-mentioned technical problems. And, other technicalproblems that are not mentioned will be apparently understood to thoseskilled in the art from the following description.

Technical Solution

A method and an apparatus for decoding a video signal according to thepresent invention may generate a plurality of Most Probable Mode (MPM)candidates, determine whether there is an MPM candidate identical to anintra-prediction mode of a current block among the plurality of MPMcandidates, obtain the intra-prediction mode of the current block, basedon a result of the determining, and perform an intra-prediction for thecurrent block, based on the intra-prediction mode of the current block.Here, the plurality of MPM candidates are generated based on occurrencefrequencies of intra-prediction modes of a plurality of neighboringblocks adjacent to the current block.

In the method and the apparatus for decoding a video signal according tothe present invention, generation of the plurality of MPM candidates maycomprise generating a first MPM candidate group, and generating a secondMPM candidate group.

In the method and the apparatus for decoding a video signal according tothe present invention, the second MPM candidate group may include an MPMcandidate having a direction similar to an MPM candidate included in thefirst MPM candidate group.

In the method and the apparatus for decoding a video signal according tothe present invention, determination of whether there is an MPMcandidate identical to an intra-prediction mode of a current block amongthe plurality of MPM candidates may comprise determining whether thefirst MPM candidate group includes an MPM candidate identical to theintra-prediction mode of the current block, and determining whether thesecond MPM candidate group includes an MPM candidate identical to theintra-prediction mode of the current block, when it is determined thatthe first MPM candidate group does not include an MPM candidateidentical to the intra-prediction mode of the current block.

In the method and the apparatus for decoding a video signal according tothe present invention, the plurality of MPM candidates may include anintra-prediction mode having a highest occurrence frequency amongintra-prediction modes of a plurality of top neighboring blocks adjacentto the current block, and an intra-prediction mode having a highestoccurrence frequency among intra-prediction modes of a plurality of leftneighboring blocks adjacent to the current block.

In the method and the apparatus for decoding a video signal according tothe present invention, the plurality of MPM candidates may include alargest or smallest intra-prediction mode among intra-prediction modesof top neighboring blocks and intra-prediction modes of neighboringblocks.

In the method and the apparatus for decoding a video signal according tothe present invention, when it is determined that there is no MPMcandidate identical to the intra-prediction mode of the current blockamong the plurality of MPM candidates, the obtaining theintra-prediction mode of the current block may comprises decoding aremaining mode, and determining the intra-prediction mode of the currentblock based on the remaining mode.

In the method and the apparatus for decoding a video signal according tothe present invention, the remaining mode may be encoded as a fixedlength.

In the method and the apparatus for decoding a video signal according tothe present invention, the performing the intra-prediction for thecurrent block may comprise selecting a reference line among a pluralityof reference lines of the current block, and obtaining a predictionsample for the current block, using the selected reference line.

In the method and the apparatus for decoding a video signal according tothe present invention, among the plurality of reference lines, an N-threference line includes a larger number of reference samples than an(N−1)-th reference sample.

A method and an apparatus for encoding a video signal according to thepresent invention may generate a plurality of Most Probable Mode (MPM)candidates, determine whether there is an MPM candidate identical to anintra-prediction mode of a current block among the plurality of MPMcandidates, and encode information indicating whether MPM candidate isincluded in the plurality of MPM candidates identical to intraprediction mode of the current block. Here, the plurality of MPMcandidates are generated based on occurrence frequencies ofintra-prediction modes of a plurality of neighboring blocks adjacent tothe current block.

The features briefly summarized above for the present invention are onlyillustrative aspects of the detailed description of the invention thatfollows, but do not limit the scope of the invention.

Advantageous Effects

According to the present invention, an efficient intra-prediction may beperformed for an encoding/decoding target block.

According to the present invention, an intra-prediction mode of anencoding/decoding target block may be obtained using a neighboring blockadjacent to an encoding/decoding target block.

According to the present invention, an intra-prediction for anencoding/decoding target block may be performed based on a plurality ofreference lines.

The effects obtainable by the present invention are not limited to theabove-mentioned effects, and other effects not mentioned can be clearlyunderstood by those skilled in the art from the description below.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a device for encoding a videoaccording to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a device for decoding a videoaccording to an embodiment of the present invention.

FIG. 3 is a view illustrating an example of hierarchically partitioninga coding block based on a tree structure according to an embodiment ofthe present invention.

FIG. 4 is a view illustrating types of pre-defined intra predictionmodes for a device for encoding/decoding a video according to anembodiment of the present invention.

FIG. 5 is a flowchart briefly illustrating an intra prediction methodaccording to an embodiment of the present invention.

FIG. 6 is a view illustrating a method of correcting a prediction sampleof a current block based on differential information of neighboringsamples according to an embodiment of the present invention.

FIGS. 7 and 8 are views illustrating a method of correcting a predictionsample based on a predetermined correction filter according to anembodiment of the present invention.

FIG. 9 is a view illustrating a method of correcting a prediction samplebased on offset according to an embodiment of the present invention.

FIGS. 10 to 14 are views illustrating examples of an intra predictionpattern of a current block according to an embodiment of the presentinvention.

FIG. 15 is a view illustrating a method of performing prediction usingan intra block copy technique according to an embodiment of the presentinvention.

FIG. 16 shows a range of reference samples for intra predictionaccording to an embodiment to which the present invention is applied.

FIGS. 17 to 19 illustrate an example of filtering on reference samples.

FIG. 20 is a flowchart illustrating a method of deriving anintra-prediction mode of a current block, according to an embodiment ofthe present invention.

FIG. 21 is a diagram for explaining an example in which an MPM candidateis generated using an intra-prediction mode of a neighboring blockadjacent to a current block.

FIGS. 22A and 22B are a diagram showing an example of generating MPMcandidates when an intra-prediction mode of a top neighboring block or aleft neighboring block adjacent to a current block is not available.

FIGS. 23A and 23B are a diagram for explaining an order of obtainingintra-prediction modes of neighboring blocks.

FIG. 24 shows an example of deriving an intra-prediction mode of acurrent block, using 2 MPM candidate groups.

FIG. 25 shows an example of deriving an intra-prediction mode of acurrent block, using 3 MPM candidate groups.

FIG. 26 exemplifies a plurality of reference sample lines.

FIG. 27 is a flowchart illustrating a method of performingintra-prediction, using an extended reference line, according to thepresent invention.

FIG. 28 is a diagram exemplifying a plurality of reference lines for anon-square block.

MODE FOR INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, and theexemplary embodiments can be construed as including all modifications,equivalents, or substitutes in a technical concept and a technical scopeof the present invention. The similar reference numerals refer to thesimilar element in described the drawings.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.Hereinafter, the same constituent elements in the drawings are denotedby the same reference numerals, and a repeated description of the sameelements will be omitted.

FIG. 1 is a block diagram illustrating a device for encoding a videoaccording to an embodiment of the present invention.

Referring to FIG. 1, the device 100 for encoding a video may include: apicture partitioning module 110, prediction modules 120 and 125, atransform module 130, a quantization module 135, a rearrangement module160, an entropy encoding module 165, an inverse quantization module 140,an inverse transform module 145, a filter module 150, and a memory 155.

The constitutional parts shown in FIG. 1 are independently shown so asto represent characteristic functions different from each other in thedevice for encoding a video. Thus, it does not mean that eachconstitutional part is constituted in a constitutional unit of separatedhardware or software. In other words, each constitutional part includeseach of enumerated constitutional parts for convenience. Thus, at leasttwo constitutional parts of each constitutional part may be combined toform one constitutional part or one constitutional part may be dividedinto a plurality of constitutional parts to perform each function. Theembodiment where each constitutional part is combined and the embodimentwhere one constitutional part is divided are also included in the scopeof the present invention, if not departing from the essence of thepresent invention.

Also, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

The picture partitioning module 110 may partition an input picture intoone or more processing units. Here, the processing unit may be aprediction unit (PU), a transform unit (TU), or a coding unit (CU). Thepicture partitioning module 110 may partition one picture intocombinations of multiple coding units, prediction units, and transformunits, and may encode a picture by selecting one combination of codingunits, prediction units, and transform units with a predeterminedcriterion (e.g., cost function).

For example, one picture may be partitioned into multiple coding units.A recursive tree structure, such as a quad tree structure, may be usedto partition a picture into coding units. A coding unit which ispartitioned into other coding units with one picture or a largest codingunit as a root may be partitioned with child nodes corresponding to thenumber of partitioned coding units. A coding unit which is no longerpartitioned by a predetermined limitation serves as a leaf node. Thatis, when it is assumed that only square partitioning is possible for onecoding unit, one coding unit may be partitioned into four other codingunits at most.

Hereinafter, in the embodiment of the present invention, the coding unitmay mean a unit performing encoding, or a unit performing decoding.

A prediction unit may be one of partitions partitioned into a square ora rectangular shape having the same size in a single coding unit, or aprediction unit may be one of partitions partitioned so as to have adifferent shape/size in a single coding unit.

When a prediction unit subjected to intra prediction is generated basedon a coding unit and the coding unit is not the smallest coding unit,intra prediction may be performed without partitioning the coding unitinto multiple prediction units N×N.

The prediction modules 120 and 125 may include an inter predictionmodule 120 performing inter prediction and an intra prediction module125 performing intra prediction. Whether to perform inter prediction orintra prediction for the prediction unit may be determined, and detailedinformation (e.g., an intra prediction mode, a motion vector, areference picture, etc.) according to each prediction method may bedetermined. Here, the processing unit subjected to prediction may bedifferent from the processing unit for which the prediction method anddetailed content is determined. For example, the prediction method, theprediction mode, etc. may be determined by the prediction unit, andprediction may be performed by the transform unit. A residual value(residual block) between the generated prediction block and an originalblock may be input to the transform module 130. Also, prediction modeinformation, motion vector information, etc. used for prediction may beencoded with the residual value by the entropy encoding module 165 andmay be transmitted to a device for decoding a video. When a particularencoding mode is used, it is possible to transmit to a device fordecoding video by encoding the original block as it is withoutgenerating the prediction block through the prediction modules 120 and125.

The inter prediction module 120 may predict the prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of the current picture, or may predict the prediction unit basedon information of some encoded regions in the current picture, in somecases. The inter prediction module 120 may include a reference pictureinterpolation module, a motion prediction module, and a motioncompensation module.

The reference picture interpolation module may receive reference pictureinformation from the memory 155 and may generate pixel information of aninteger pixel or less then the integer pixel from the reference picture.In the case of luma pixels, an 8-tap DCT-based interpolation filterhaving different filter coefficients may be used to generate pixelinformation of an integer pixel or less than an integer pixel in unitsof a ¼ pixel. In the case of chroma signals, a 4-tap DCT-basedinterpolation filter having different filter coefficient may be used togenerate pixel information of an integer pixel or less than an integerpixel in units of a ⅛ pixel.

The motion prediction module may perform motion prediction based on thereference picture interpolated by the reference picture interpolationmodule. As methods for calculating a motion vector, various methods,such as a full search-based block matching algorithm (FBMA), a threestep search (TSS), a new three-step search algorithm (NTS), etc., may beused. The motion vector may have a motion vector value in units of a ½pixel or a ¼ pixel based on an interpolated pixel. The motion predictionmodule may predict a current prediction unit by changing the motionprediction method. As motion prediction methods, various methods, suchas a skip method, a merge method, an AMVP (Advanced Motion VectorPrediction) method, an intra block copy method, etc., may be used.

The intra prediction module 125 may generate a prediction unit based onreference pixel information neighboring to a current block which ispixel information in the current picture. When the neighboring block ofthe current prediction unit is a block subjected to inter prediction andthus a reference pixel is a pixel subjected to inter prediction, thereference pixel included in the block subjected to inter prediction maybe replaced with reference pixel information of a neighboring blocksubjected to intra prediction. That is, when a reference pixel is notavailable, at least one reference pixel of available reference pixelsmay be used instead of unavailable reference pixel information.

Prediction modes in intra prediction may include a directionalprediction mode using reference pixel information depending on aprediction direction and a non-directional prediction mode not usingdirectional information in performing prediction. A mode for predictingluma information may be different from a mode for predicting chromainformation, and in order to predict the chroma information, intraprediction mode information used to predict luma information orpredicted luma signal information may be utilized.

In performing intra prediction, when the size of the prediction unit isthe same as the size of the transform unit, intra prediction may beperformed on the prediction unit based on pixels positioned at the left,the top left, and the top of the prediction unit. However, in performingintra prediction, when the size of the prediction unit is different fromthe size of the transform unit, intra prediction may be performed usinga reference pixel based on the transform unit. Also, intra predictionusing N×N partitioning may be used for only the smallest coding unit.

In the intra prediction method, a prediction block may be generatedafter applying an AIS (Adaptive Intra Smoothing) filter to a referencepixel depending on the prediction modes. The type of the AIS filterapplied to the reference pixel may vary. In order to perform the intraprediction method, an intra prediction mode of the current predictionunit may be predicted from the intra prediction mode of the predictionunit neighboring to the current prediction unit. In prediction of theprediction mode of the current prediction unit by using mode informationpredicted from the neighboring prediction unit, when the intraprediction mode of the current prediction unit is the same as the intraprediction mode of the neighboring prediction unit, informationindicating that the prediction modes of the current prediction unit andthe neighboring prediction unit are equal to each other may betransmitted using predetermined flag information. When the predictionmode of the current prediction unit is different from the predictionmode of the neighboring prediction unit, entropy encoding may beperformed to encode prediction mode information of the current block.

Also, a residual block including information on a residual value whichis a different between the prediction unit subjected to prediction andthe original block of the prediction unit may be generated based onprediction units generated by the prediction modules 120 and 125. Thegenerated residual block may be input to the transform module 130.

The transform module 130 may transform the residual block including theinformation on the residual value between the original block and theprediction unit generated by the prediction modules 120 and 125 by usinga transform method, such as discrete cosine transform (DCT), discretesine transform (DST), and KLT. Whether to apply DCT, DST, or KLT inorder to transform the residual block may be determined based on intraprediction mode information of the prediction unit used to generate theresidual block.

The quantization module 135 may quantize values transformed to afrequency domain by the transform module 130. Quantization coefficientsmay vary depending on the block or importance of a picture. The valuescalculated by the quantization module 135 may be provided to the inversequantization module 140 and the rearrangement module 160.

The rearrangement module 160 may rearrange coefficients of quantizedresidual values.

The rearrangement module 160 may change a coefficient in the form of atwo-dimensional block into a coefficient in the form of aone-dimensional vector through a coefficient scanning method. Forexample, the rearrangement module 160 may scan from a DC coefficient toa coefficient in a high frequency domain using a zigzag scanning methodso as to change the coefficients to be in the form of one-dimensionalvectors. Depending on the size of the transform unit and the intraprediction mode, vertical direction scanning where coefficients in theform of two-dimensional blocks are scanned in the column direction orhorizontal direction scanning where coefficients in the form oftwo-dimensional blocks are scanned in the row direction may be usedinstead of zigzag scanning. That is, which scanning method among zigzagscanning, vertical direction scanning, and horizontal direction scanningis used may be determined depending on the size of the transform unitand the intra prediction mode.

The entropy encoding module 165 may perform entropy encoding based onthe values calculated by the rearrangement module 160. Entropy encodingmay use various encoding methods, for example, exponential Golombcoding, context-adaptive variable length coding (CAVLC), andcontext-adaptive binary arithmetic coding (CABAC).

The entropy encoding module 165 may encode a variety of information,such as residual value coefficient information and block typeinformation of the coding unit, prediction mode information, partitionunit information, prediction unit information, transform unitinformation, motion vector information, reference frame information,block interpolation information, filtering information, etc. from therearrangement module 160 and the prediction modules 120 and 125.

The entropy encoding module 165 may entropy encode the coefficients ofthe coding unit input from the rearrangement module 160.

The inverse quantization module 140 may inversely quantize the valuesquantized by the quantization module 135 and the inverse transformmodule 145 may inversely transform the values transformed by thetransform module 130. The residual value generated by the inversequantization module 140 and the inverse transform module 145 may becombined with the prediction unit predicted by a motion estimationmodule, a motion compensation module, and the intra prediction module ofthe prediction modules 120 and 125 such that a reconstructed block canbe generated.

The filter module 150 may include at least one of a deblocking filter,an offset correction unit, and an adaptive loop filter (ALF).

The deblocking filter may remove block distortion that occurs due toboundaries between the blocks in the reconstructed picture. In order todetermine whether to perform deblocking, the pixels included in severalrows or columns in the block may be a basis of determining whether toapply the deblocking filter to the current block. When the deblockingfilter is applied to the block, a strong filter or a weak filter may beapplied depending on required deblocking filtering strength. Also, inapplying the deblocking filter, horizontal direction filtering andvertical direction filtering may be processed in parallel.

The offset correction module may correct offset with the originalpicture in units of a pixel in the picture subjected to deblocking. Inorder to perform the offset correction on a particular picture, it ispossible to use a method of applying offset in consideration of edgeinformation of each pixel or a method of partitioning pixels of apicture into the predetermined number of regions, determining a regionto be subjected to perform offset, and applying the offset to thedetermined region.

Adaptive loop filtering (ALF) may be performed based on the valueobtained by comparing the filtered reconstructed picture and theoriginal picture. The pixels included in the picture may be divided intopredetermined groups, a filter to be applied to each of the groups maybe determined, and filtering may be individually performed for eachgroup. Information on whether to apply ALF and a luma signal may betransmitted by coding units (CU). The shape and filter coefficient of afilter for ALF may vary depending on each block. Also, the filter forALF in the same shape (fixed shape) may be applied regardless ofcharacteristics of the application target block.

The memory 155 may store the reconstructed block or picture calculatedthrough the filter module 150. The stored reconstructed block or picturemay be provided to the prediction modules 120 and 125 in performinginter prediction.

FIG. 2 is a block diagram illustrating a device for decoding a videoaccording to an embodiment of the present invention.

Referring to FIG. 2, the device 200 for decoding a video may include: anentropy decoding module 210, a rearrangement module 215, an inversequantization module 220, an inverse transform module 225, predictionmodules 230 and 235, a filter module 240, and a memory 245.

When a video bitstream is input from the device for encoding a video,the input bitstream may be decoded according to an inverse process ofthe device for encoding a video.

The entropy decoding module 210 may perform entropy decoding accordingto an inverse process of entropy encoding by the entropy encoding moduleof the device for encoding a video. For example, corresponding to themethods performed by the device for encoding a video, various methods,such as exponential Golomb coding, context-adaptive variable lengthcoding (CAVLC), and context-adaptive binary arithmetic coding (CABAC)may be applied.

The entropy decoding module 210 may decode information on intraprediction and inter prediction performed by the device for encoding avideo.

The rearrangement module 215 may perform rearrangement on the bitstreamentropy decoded by the entropy decoding module 210 based on therearrangement method used in the device for encoding a video. Therearrangement module may reconstruct and rearrange the coefficients inthe form of one-dimensional vectors to the coefficient in the form oftwo-dimensional blocks. The rearrangement module 215 may receiveinformation related to coefficient scanning performed in the device forencoding a video and may perform rearrangement via a method of inverselyscanning the coefficients based on the scanning order performed in thedevice for encoding a video.

The inverse quantization module 220 may perform inverse quantizationbased on a quantization parameter received from the device for encodinga video and the rearranged coefficients of the block.

The inverse transform module 225 may perform the inverse transform,i.e., inverse DCT, inverse DST, and inverse KLT, which is the inverseprocess of transform, i.e., DCT, DST, and KLT, performed by thetransform module on the quantization result by the device for encoding avideo. Inverse transform may be performed based on a transfer unitdetermined by the device for encoding a video. The inverse transformmodule 225 of the device for decoding a video may selectively performtransform schemes (e.g., DCT, DST, and KLT) depending on multiple piecesof information, such as the prediction method, the size of the currentblock, the prediction direction, etc.

The prediction modules 230 and 235 may generate a prediction block basedon information on prediction block generation received from the entropydecoding module 210 and previously decoded block or picture informationreceived from the memory 245.

As described above, like the operation of the device for encoding avideo, in performing intra prediction, when the size of the predictionunit is the same as the size of the transform unit, intra prediction maybe performed on the prediction unit based on the pixels positioned atthe left, the top left, and the top of the prediction unit. Inperforming intra prediction, when the size of the prediction unit isdifferent from the size of the transform unit, intra prediction may beperformed using a reference pixel based on the transform unit. Also,intra prediction using N×N partitioning may be used for only thesmallest coding unit.

The prediction modules 230 and 235 may include a prediction unitdetermination module, an inter prediction module, and an intraprediction module. The prediction unit determination module may receivea variety of information, such as prediction unit information,prediction mode information of an intra prediction method, informationon motion prediction of an inter prediction method, etc. from theentropy decoding module 210, may divide a current coding unit intoprediction units, and may determine whether inter prediction or intraprediction is performed on the prediction unit. By using informationrequired in inter prediction of the current prediction unit receivedfrom the device for encoding a video, the inter prediction module 230may perform inter prediction on the current prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of the current picture including the current prediction unit.Alternatively, inter prediction may be performed based on information ofsome pre-reconstructed regions in the current picture including thecurrent prediction unit.

In order to perform inter prediction, it may be determined for thecoding unit which of a skip mode, a merge mode, an AMVP mode, and aninter block copy mode is used as the motion prediction method of theprediction unit included in the coding unit.

The intra prediction module 235 may generate a prediction block based onpixel information in the current picture. When the prediction unit is aprediction unit subjected to intra prediction, intra prediction may beperformed based on intra prediction mode information of the predictionunit received from the device for encoding a video. The intra predictionmodule 235 may include an adaptive intra smoothing (AIS) filter, areference pixel interpolation module, and a DC filter. The AIS filterperforms filtering on the reference pixel of the current block, andwhether to apply the filter may be determined depending on theprediction mode of the current prediction unit. AIS filtering may beperformed on the reference pixel of the current block by using theprediction mode of the prediction unit and AIS filter informationreceived from the device for encoding a video. When the prediction modeof the current block is a mode where AIS filtering is not performed, theAIS filter may not be applied.

When the prediction mode of the prediction unit is a prediction mode inwhich intra prediction is performed based on the pixel value obtained byinterpolating the reference pixel, the reference pixel interpolationmodule may interpolate the reference pixel to generate the referencepixel of an integer pixel or less than an integer pixel. When theprediction mode of the current prediction unit is a prediction mode inwhich a prediction block is generated without interpolation thereference pixel, the reference pixel may not be interpolated. The DCfilter may generate a prediction block through filtering when theprediction mode of the current block is a DC mode.

The reconstructed block or picture may be provided to the filter module240. The filter module 240 may include the deblocking filter, the offsetcorrection module, and the ALF.

Information on whether or not the deblocking filter is applied to thecorresponding block or picture and information on which of a strongfilter and a weak filter is applied when the deblocking filter isapplied may be received from the device for encoding a video. Thedeblocking filter of the device for decoding a video may receiveinformation on the deblocking filter from the device for encoding avideo, and may perform deblocking filtering on the corresponding block.

The offset correction module may perform offset correction on thereconstructed picture based on the type of offset correction and offsetvalue information applied to a picture in performing encoding.

The ALF may be applied to the coding unit based on information onwhether to apply the ALF, ALF coefficient information, etc. receivedfrom the device for encoding a video. The ALF information may beprovided as being included in a particular parameter set.

The memory 245 may store the reconstructed picture or block for use as areference picture or block, and may provide the reconstructed picture toan output module.

As described above, in the embodiment of the present invention, forconvenience of explanation, the coding unit is used as a termrepresenting a unit for encoding, but the coding unit may serve as aunit performing decoding as well as encoding.

FIG. 3 is a view illustrating an example of hierarchically partitioninga coding block based on a tree structure according to an embodiment ofthe present invention.

An input video signal is decoded in predetermined block units. Such adefault unit for decoding the input video signal is a coding block. Thecoding block may be a unit performing intra/inter prediction, transform,and quantization. The coding block may be a square or non-square blockhaving an arbitrary size in a range of 8×8 to 64×64, or may be a squareor non-square block having a size of 128×128, 256×256, or more.

Specifically, the coding block may be hierarchically partitioned basedon at least one of a quad tree and a binary tree. Here, quad tree-basedpartitioning may mean that a 2N×2N coding block is partitioned into fourN×N coding blocks, and binary tree-based partitioning may mean that onecoding block is partitioned into two coding blocks. Binary tree-basedpartitioning may be symmetrically or asymmetrically performed. Thecoding block partitioned based on the binary tree may be a square blockor a non-square block, such as a rectangular shape. Binary tree-basedpartitioning may be performed on a coding block where quad tree-basedpartitioning is no longer performed. Quad tree-based partitioning may nolonger be performed on the coding block partitioned based on the binarytree.

In order to implement adaptive partitioning based on the quad tree orbinary tree, information indicating quad tree-based partitioning,information on the size/depth of the coding block that quad tree-basedpartitioning is allowed, information indicating binary tree-basedpartitioning, information on the size/depth of the coding block thatbinary tree-based partitioning is allowed, information on the size/depthof the coding block that binary tree-based partitioning is not allowed,information on whether binary tree-based partitioning is performed in avertical direction or a horizontal direction, etc. may be used.

As shown in FIG. 3, the first coding block 300 with the partition depth(split depth) of k may be partitioned into multiple second coding blocksbased on the quad tree. For example, the second coding blocks 310 to 340may be square blocks having the half width and the half height of thefirst coding block, and the partition depth of the second coding blockmay be increased to k+1.

The second coding block 310 with the partition depth of k+1 may bepartitioned into multiple third coding blocks with the partition depthof k+2. Partitioning of the second coding block 310 may be performed byselectively using one of the quad tree and the binary tree depending ona partitioning method. Here, the partitioning method may be determinedbased on at least one of the information indicating quad tree-basedpartitioning and the information indicating binary tree-basedpartitioning.

When the second coding block 310 is partitioned based on the quad tree,the second coding block 310 may be partitioned into four third codingblocks 310 a having the half width and the half height of the secondcoding block, and the partition depth of the third coding block 310 amay be increased to k+2. In contrast, when the second coding block 310is partitioned based on the binary tree, the second coding block 310 maybe partitioned into two third coding blocks. Here, each of two thirdcoding blocks may be a non-square block having one of the half width andthe half height of the second coding block, and the partition depth maybe increased to k+2. The second coding block may be determined as anon-square block of a horizontal direction or a vertical directiondepending on a partitioning direction, and the partitioning directionmay be determined based on the information on whether binary tree-basedpartitioning is performed in a vertical direction or a horizontaldirection.

In the meantime, the second coding block 310 may be determined as a leafcoding block that is no longer partitioned based on the quad tree or thebinary tree. In this case, the leaf coding block may be used as aprediction block or a transform block.

Like partitioning of the second coding block 310, the third coding block310 a may be determined as a leaf coding block, or may be furtherpartitioned based on the quad tree or the binary tree.

In the meantime, the third coding block 310 b partitioned based on thebinary tree may be further partitioned into coding blocks 310 b-2 of avertical direction or coding blocks 310 b-3 of a horizontal directionbased on the binary tree, and the partition depth of the relevant codingblocks may be increased to k+3. Alternatively, the third coding block310 b may be determined as a leaf coding block 310 b-1 that is no longerpartitioned based on the binary tree. In this case, the coding block 310b-1 may be used as a prediction block or a transform block. However, theabove partitioning process may be limitedly performed based on at leastone of the information on the size/depth of the coding block that quadtree-based partitioning is allowed, the information on the size/depth ofthe coding block that binary tree-based partitioning is allowed, and theinformation on the size/depth of the coding block that binary tree-basedpartitioning is not allowed.

FIG. 4 is a view illustrating types of pre-defined intra predictionmodes for a device for encoding/decoding a video according to anembodiment of the present invention.

The device for encoding/decoding a video may perform intra predictionusing one of pre-defined intra prediction modes. The pre-defined intraprediction modes for intra prediction may include non-directionalprediction modes (e.g., a planar mode, a DC mode) and 33 directionalprediction modes.

Alternatively, in order to enhance accuracy of intra prediction, alarger number of directional prediction modes than the 33 directionalprediction modes may be used. That is, M extended directional predictionmodes may be defined by subdividing angles of the directional predictionmodes (M>33), and a directional prediction mode having a predeterminedangle may be derived using at least one of the 33 pre-defineddirectional prediction modes.

FIG. 4 shows an example of extended intra prediction modes, and theextended intra prediction modes may include two non-directionalprediction modes and 65 extended directional prediction modes. The samenumbers of the extended intra prediction modes may be used for a lumacomponent and a chroma component, or a different number of intraprediction modes may be used for each component. For example, 67extended intra prediction modes may be used for the luma component, and35 intra prediction modes may be used for the chroma component.

Alternatively, depending on the chroma format, a different number ofintra prediction modes may be used in performing intra prediction. Forexample, in the case of the 4:2:0 format, 67 intra prediction modes maybe used for the luma component to perform intra prediction and 35 intraprediction modes may be used for the chroma component. In the case ofthe 4:4:4 format, 67 intra prediction modes may be used for both theluma component and the chroma component to perform intra prediction.

Alternatively, depending on the size and/or shape of the block, adifferent number of intra prediction modes may be used to perform intraprediction. That is, depending on the size and/or shape of the PU or CU,35 intra prediction modes or 67 intra prediction modes may be used toperform intra prediction. For example, when the CU or PU has the sizeless than 64×64 or is asymmetrically partitioned, 35 intra predictionmodes may be used to perform intra prediction. When the size of the CUor PU is equal to or greater than 64×64, 67 intra prediction modes maybe used to perform intra prediction. 65 directional intra predictionmodes may be allowed for Intra 2N×2N, and only 35 directional intraprediction modes may be allowed for Intra N×N.

FIG. 5 is a flowchart briefly illustrating an intra prediction methodaccording to an embodiment of the present invention.

Referring to FIG. 5, an intra prediction mode of the current block maybe determined at step S500.

Specifically, the intra prediction mode of the current block may bederived based on a candidate list and an index. Here, the candidate listcontains multiple candidates, and the multiple candidates may bedetermined based on an intra prediction mode of the neighboring blockadjacent to the current block. The neighboring block may include atleast one of blocks positioned at the top, the bottom, the left, theright, and the corner of the current block. The index may specify one ofthe multiple candidates of the candidate list. The candidate specifiedby the index may be set to the intra prediction mode of the currentblock.

An intra prediction mode used for intra prediction in the neighboringblock may be set as a candidate. Also, an intra prediction mode havingdirectionality similar to that of the intra prediction mode of theneighboring block may be set as a candidate. Here, the intra predictionmode having similar directionality may be determined by adding orsubtracting a predetermined constant value to or from the intraprediction mode of the neighboring block. The predetermined constantvalue may be an integer, such as one, two, or more.

The candidate list may further include a default mode. The default modemay include at least one of a planar mode, a DC mode, a vertical mode,and a horizontal mode. The default mode may be adaptively addedconsidering the maximum number of candidates that can be included in thecandidate list of the current block.

The maximum number of candidates that can be included in the candidatelist may be three, four, five, six, or more. The maximum number ofcandidates that can be included in the candidate list may be a fixedvalue preset in the device for encoding/decoding a video, or may bevariably determined based on a characteristic of the current block. Thecharacteristic may mean the location/size/shape of the block, thenumber/type of intra prediction modes that the block can use, etc.Alternatively, information indicating the maximum number of candidatesthat can be included in the candidate list may be signaled separately,and the maximum number of candidates that can be included in thecandidate list may be variably determined using the information. Theinformation indicating the maximum number of candidates may be signaledin at least one of a sequence level, a picture level, a slice level, anda block level.

When the extended intra prediction modes and the 35 pre-defined intraprediction modes are selectively used, the intra prediction modes of theneighboring blocks may be transformed into indexes corresponding to theextended intra prediction modes, or into indexes corresponding to the 35intra prediction modes, whereby candidates can be derived. For transformto an index, a pre-defined table may be used, or a scaling operationbased on a predetermined value may be used. Here, the pre-defined tablemay define a mapping relation between different intra prediction modegroups (e.g., extended intra prediction modes and 35 intra predictionmodes).

For example, when the left neighboring block uses the 35 intraprediction modes and the intra prediction mode of the left neighboringblock is 10 (a horizontal mode), it may be transformed into an index of16 corresponding to a horizontal mode in the extended intra predictionmodes.

Alternatively, when the top neighboring block uses the extended intraprediction modes and the intra prediction mode the top neighboring blockhas an index of 50 (a vertical mode), it may be transformed into anindex of 26 corresponding to a vertical mode in the 35 intra predictionmodes.

Based on the above-described method of determining the intra predictionmode, the intra prediction mode may be derived independently for each ofthe luma component and the chroma component, or the intra predictionmode of the chroma component may be derived depending on the intraprediction mode of the luma component.

Specifically, the intra prediction mode of the chroma component may bedetermined based on the intra prediction mode of the luma component asshown in the following Table 1.

TABLE 1 IntraPredModeY[xCb][yCb] X(0 <= Intra_chroma_pred_mode[xCb][yCb]0 26 10 1 X <= 34) 0 34 0 0 0 0 1 26 34 26 26 26 2 10 10 34 10 10 3 1 11 34 1 4 0 26 10 1 X

In Table 1, intra_chroma_pred_mode means information signaled to specifythe intra prediction mode of the chroma component, and IntraPredModeYindicates the intra prediction mode of the luma component.

Referring to FIG. 5, a reference sample for intra prediction of thecurrent block may be derived at step S510.

Specifically, a reference sample for intra prediction may be derivedbased on a neighboring sample of the current block. The neighboringsample may be a reconstructed sample of the neighboring block, and thereconstructed sample may be a reconstructed sample before an in-loopfilter is applied or a reconstructed sample after the in-loop filter isapplied.

A neighboring sample reconstructed before the current block may be usedas the reference sample, and a neighboring sample filtered based on apredetermined intra filter may be used as the reference sample. Theintra filter may include at least one of the first intra filter appliedto multiple neighboring samples positioned on the same horizontal lineand the second intra filter applied to multiple neighboring samplespositioned on the same vertical line. Depending on the positions of theneighboring samples, one of the first intra filter and the second intrafilter may be selectively applied, or both intra filters may be applied.

Filtering may be adaptively performed based on at least one of the intraprediction mode of the current block and the size of the transform blockfor the current block. For example, when the intra prediction mode ofthe current block is the DC mode, the vertical mode, or the horizontalmode, filtering may not be performed. When the size of the transformblock is N×M, filtering may not be performed. Here, N and M may be thesame values or different values, or may be values of 4, 8, 16, or more.Alternatively, filtering may be selectively performed based on theresult of a comparison of a pre-defined threshold and the differencebetween the intra prediction mode of the current block and the verticalmode (or the horizontal mode). For example, when the difference betweenthe intra prediction mode of the current block and the vertical mode isgreater than a threshold, filtering may be performed. The threshold maybe defined for each size of the transform block as shown in Table 2.

TABLE 2 8 × 8 transform 16 × 16 transform 32 × 32 transform Threshold 71 0

The intra filter may be determined as one of multiple intra filtercandidates pre-defined in the device for encoding/decoding a video. Tothis end, an index specifying an intra filter of the current block amongthe multiple intra filter candidates may be signaled. Alternatively, theintra filter may be determined based on at least one of the size/shapeof the current block, the size/shape of the transform block, informationon the filter strength, and variations of the neighboring samples.

Referring to FIG. 5, intra prediction may be performed using the intraprediction mode of the current block and the reference sample at stepS520.

That is, the prediction sample of the current block may be obtainedusing the intra prediction mode determined at step S500 and thereference sample derived at step S510. However, in the case of intraprediction, a boundary sample of the neighboring block may be used, andthus quality of the prediction picture may be decreased. Therefore, acorrection process may be performed on the prediction sample generatedthrough the above-described prediction process, and will be described indetail with reference to FIGS. 6 to 14. However, the correction processis not limited to being applied only to the intra prediction sample, andmay be applied to an inter prediction sample or the reconstructedsample.

FIG. 6 is a view illustrating a method of correcting a prediction sampleof a current block based on differential information of neighboringsamples according to an embodiment of the present invention.

The prediction sample of the current block may be corrected based on thedifferential information of multiple neighboring samples for the currentblock. The correction may be performed on all prediction samples in thecurrent block, or may be performed on prediction samples in somepredetermined regions. Some regions may be one row/column or multiplerows/columns, or may be preset regions for correction in the device forencoding/decoding a video, or may be variably determined based on atleast one of the size/shape of the current block and the intraprediction mode.

The neighboring samples may belong to the neighboring blocks positionedat the top, the left, and the top left corner of the current block. Thenumber of neighboring samples used for correction may be two, three,four, or more. The positions of the neighboring samples may be variablydetermined depending on the position of the prediction sample which isthe correction target in the current block. Alternatively, some of theneighboring samples may have fixed positions regardless of the positionof the prediction sample which is the correction target, and theremaining neighboring samples may have variable positions depending onthe position of the prediction sample which is the correction target.

The differential information of the neighboring samples may mean adifferential sample between the neighboring samples, or may mean a valueobtained by scaling the differential sample by a predetermined constantvalue (e.g., one, two, three, etc.). Here, the predetermined constantvalue may be determined considering the position of the predictionsample which is the correction target, the position of the column or rowincluding the prediction sample which is the correction target, theposition of the prediction sample within the column or row, etc.

For example, when the intra prediction mode of the current block is thevertical mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p (−1, y) adjacent to the leftboundary of the current block may be used to obtain the final predictionsample as shown in Equation 1.P′(0,y)=P(0,y)+((p(−1,y)−p(−1,−1))>>1 for y=0 . . . N−1  [Equation 1]

For example, when the intra prediction mode of the current block is thehorizontal mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(x, −1) adjacent to the topboundary of the current block may be used to obtain the final predictionsample as shown in Equation 2.P′(x,0)=p(x,0)+((p(x,−1)−p(−1,−1))>>1 for x=0 . . . N−1  [Equation 2]

For example, when the intra prediction mode of the current block is thevertical mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(−1, y) adjacent to the leftboundary of the current block may be used to obtain the final predictionsample. Here, the differential sample may be added to the predictionsample, or the differential sample may be scaled by a predeterminedconstant value, and then added to the prediction sample. Thepredetermined constant value used in scaling may be determineddifferently depending on the column and/or row. For example, theprediction sample may be corrected as shown in Equation 3 and Equation4.P′(0,y)=P(0,y)+((p(−1,y)−p(−1,−1))>>1 for y=0 . . . N−1  [Equation 3]P′(1,y)=P(1,y)+((p(−1,y)−p(−1,−1))>>2 for y=0 . . . N−1  [Equation 4]

For example, when the intra prediction mode of the current block is thehorizontal mode, differential samples between the top left neighboringsample p(−1, −1) and neighboring samples p(x, −1) adjacent to the topboundary of the current block may be used to obtain the final predictionsample, as described in the case of the vertical mode. For example, theprediction sample may be corrected as shown in Equation 5 and Equation6.P′(x,0)=p(x,0)+((p(x,−1)−p(−1,−1))>>1 for x=0 . . . N−1  [Equation 5]P′(x,1)=p(x,1)+((p(x,−1)−p(−1,−1))>>2 for x=0 . . . N−1  [Equation 6]

FIGS. 7 and 8 are views illustrating a method of correcting a predictionsample based on a predetermined correction filter according to anembodiment of the present invention.

The prediction sample may be corrected based on the neighboring sampleof the prediction sample which is the correction target and apredetermined correction filter. Here, the neighboring sample may bespecified by an angular line of the directional prediction mode of thecurrent block, or may be at least one sample positioned on the sameangular line as the prediction sample which is the correction target.Also, the neighboring sample may be a prediction sample in the currentblock, or may be a reconstructed sample in a neighboring blockreconstructed before the current block.

At least one of the number of taps, strength, and a filter coefficientof the correction filter may be determined based on at least one of theposition of the prediction sample which is the correction target,whether or not the prediction sample which is the correction target ispositioned on the boundary of the current block, the intra predictionmode of the current block, angle of the directional prediction mode, theprediction mode (inter or intra mode) of the neighboring block, and thesize/shape of the current block.

Referring to FIG. 7, when the directional prediction mode has an indexof 2 or 34, at least one prediction/reconstructed sample positioned atthe bottom left of the prediction sample which is the correction targetand the predetermined correction filter may be used to obtain the finalprediction sample. Here, the prediction/reconstructed sample at thebottom left may belong to a previous line of a line including theprediction sample which is the correction target. Theprediction/reconstructed sample at the bottom left may belong to thesame block as the current sample, or to neighboring block adjacent tothe current block.

Filtering for the prediction sample may be performed only on the linepositioned at the block boundary, or may be performed on multiple lines.The correction filter where at least one of the number of filter tapsand a filter coefficient is different for each of lines may be used. Forexample, a (½, ½) filter may be used for the left first line closest tothe block boundary, a ( 12/16, 4/16) filter may be used for the secondline, a ( 14/16, 2/16) filter may be used for the third line, and a (15/16, 1/16) filter may be used for the fourth line.

Alternatively, when the directional prediction mode has an index of 3 to6 or 30 to 33, filtering may be performed on the block boundary as shownin FIG. 8, and a 3-tap correction filter may be used to correct theprediction sample. Filtering may be performed using the bottom leftsample of the prediction sample which is the correction target, thebottom sample of the bottom left sample, and a 3-tap correction filterthat takes as input the prediction sample which is the correctiontarget. The position of neighboring sample used by the correction filtermay be determined differently based on the directional prediction mode.The filter coefficient of the correction filter may be determineddifferently depending on the directional prediction mode.

Different correction filters may be applied depending on whether theneighboring block is encoded in the inter mode or the intra mode. Whenthe neighboring block is encoded in the intra mode, a filtering methodwhere more weight is given to the prediction sample may be used,compared to when the neighboring block is encoded in the inter mode. Forexample, in the case of that the intra prediction mode is 34, when theneighboring block is encoded in the inter mode, a (½, ½) filter may beused, and when the neighboring block is encoded in the intra mode, a (4/16, 12/16) filter may be used.

The number of lines to be filtered in the current block may varydepending on the size/shape of the current block (e.g., the coding blockor the prediction block). For example, when the size of the currentblock is equal to or less than 32×32, filtering may be performed on onlyone line at the block boundary; otherwise, filtering may be performed onmultiple lines including the one line at the block boundary.

FIGS. 7 and 8 are based on the case where the 35 intra prediction modesin FIG. 4 are used, but may be equally/similarly applied to the casewhere the extended intra prediction modes are used.

When performing intra prediction on a current block based on adirectional intra prediction mode, a generated prediction sample may notreflect the characteristics of an original picture since a range ofreference samples being used is limited (e.g., intra prediction isperformed only using the neighboring samples adjacent to the currentblock). For example, when an edge exists in a current block or when anew object appears around a boundary of the current block, a differencebetween a prediction sample and an original picture may be largedepending on a position of a prediction sample in the current block.

In this case, a residual value is relatively large, and thus the numberof bits to be encoded/decoded may increase. Particularly, a residualvalue in a region relatively far from a boundary of the current blockmay include a large amount of high-frequency components, which mayresult in degradation of encoding/decoding efficiency.

In order to solve the above problems, a method of generating or updatinga prediction sample in units of sub-block may be used. According tothis, prediction accuracy in a region relatively far from a blockboundary may be enhanced.

For convenience of explanation, in the following embodiments, aprediction sample generated based on a directional intra prediction modeis referred to as a first prediction sample. Also, a prediction samplegenerated based on a non-directional intra prediction mode or aprediction sample generated by performing inter prediction may also beincluded in a category of the first prediction sample.

A method of correcting the prediction sample based on the offset will bedescribed in detail with reference to FIG. 9.

FIG. 16 is a view illustrating a method of correcting a predictionsample based on offset according to an embodiment of the presentinvention.

Referring to FIG. 9, for a current block, whether to update a firstprediction sample using an offset may be determined at step S900.Whether to update the first prediction sample using the offset may bedetermined by a flag decoded from a bitstream. For example, a syntax‘is_sub_block_refinement_flag’ indicating whether to update the firstprediction sample using the offset may be signaled through a bitstream.When a value of is_sub_block_refinement_flag is 1, the method ofupdating the first prediction sample using the offset may be used in thecurrent block. When a value of is_sub_block_refinement_flag is 0, themethod of updating the first prediction sample using the offset may notbe used in the current block. However, step S900 is intended toselectively perform updating of the first prediction sample, and is notan essential configuration for achieving the purpose of the presentinvention, so that step S900 may be omitted in some cases.

When it is determined that the method of updating the first predictionsample using the offset is used, an intra prediction pattern of thecurrent block may be determined at step S910. Through the intraprediction pattern, all or some regions of the current block to whichthe offset is applied, a partition type of the current block, whether toapply the offset to a sub-block included in the current block, asize/sign of the offset assigned to each sub-block, etc. may bedetermined.

One of multiple patterns pre-defined in the device for encoding/decodinga video may be selectively used as the intra prediction pattern of thecurrent block, and to this end, an index specifying the intra predictionpattern of the current block may be signaled from a bitstream. Asanother example, the intra prediction pattern of the current block maybe determined based on a partition mode of a prediction unit or a codingunit of the current block, a size/shape of the block, whether thedirectional intra prediction mode is used, an angle of the directionalintra prediction mode, etc.

It is determined by predetermined flag information signaled via abitstream whether or not an index indicating the intra predictionpattern of the current block is signaled. For example, when the flaginformation indicates that index indicating the intra prediction patternof the current block is signaled from a bitstream, the intra predictionpattern of the current block may be determined based on an index decodedfrom a bitstream. Here, the flag information may be signaled in at leastone of a picture level, a slice level, and a block level.

When the flag information indicates that the index indicating the intraprediction pattern of the current block is not signaled from abitstream, the intra prediction pattern of the current block may bedetermined based on the partition mode of the prediction unit or thecoding unit of the current block, etc. For example, the pattern in whichthe current block is partitioned into sub-blocks may be the same as thepattern in which the coding block is partitioned into prediction units.

When the intra prediction pattern of the current block is determined,the offset may be obtained in units of sub-block at step S920. Theoffset may be signaled in units of a slice, a coding unit, or aprediction unit. As another example, the offset may be derived from aneighboring sample of the current block. The offset may include at leastone of offset value information and offset sign information. Here, theoffset value information may be in a range of integers equal to orgreater than zero.

When the offset is determined, a second prediction sample may beobtained for each sub-block at step S930. The second prediction samplemay be obtained by applying the offset to the first prediction sample.For example, the second prediction sample may be obtained adding orsubtracting the offset to or from the first prediction sample.

FIGS. 10 to 14 are views illustrating examples of an intra predictionpattern of a current block according to an embodiment of the presentinvention.

For example, in the example shown in FIG. 10, when the index is ‘0’ or‘1’, the current block may be partitioned into upper and lowersub-blocks. The offset may be not set to the upper sub-block, and theoffset ‘f’ may be set to the lower sub-block. Therefore, the firstprediction sample (P(i,j)) may be used as it is in the upper sub-block,and the second prediction sample (P(i,j)+f or P(i,j)−f) that isgenerated by adding or subtracting the offset to or from the firstprediction sample may be used in the lower sub-block. In the presentinvention, ‘not set’ may mean that the offset is not assigned to theblock, or the offset having the value of ‘0’ may be assigned to theblock.

When the index is ‘2’ or ‘3’, the current block is partitioned into leftand right sub-blocks. The offset may not be set for the left sub-block,and the offset ‘f’ may be set for the right sub-block. Therefore, thefirst prediction sample (P(i,j)) may be used as it is in the leftsub-block, and the second prediction sample (P(i,j)+f or P(i,j)−f) thatis generated adding or subtracting the offset to or from the firstprediction sample may be used in the right sub-block.

The range of available intra prediction patterns may be limited based onthe intra prediction mode of the current block. For example, when theintra prediction mode of the current block is a vertical direction intraprediction mode or a prediction mode in a direction similar to thevertical direction intra prediction mode (e.g., among the 33 directionalprediction modes, when the intra prediction mode has an index of 22 to30), only the intra prediction pattern partitioning the current block ina horizontal direction (e.g., the index 0 or index 1 in FIG. 17) may beapplied to the current block.

As another example, when the intra prediction mode of the current blockis a horizontal direction intra prediction mode or a prediction mode ina direction similar to the horizontal direction intra prediction mode(e.g., among the 33 directional prediction modes, when the intraprediction mode has an index of 6 to 14), only the intra predictionpattern partitioning the current block in a vertical direction (e.g.,the index 2 or index 3 in FIG. 17) may be applied to the current block.

In FIG. 10, the offset is not set for one of the sub-blocks included inthe current block, but it is set for the other. Whether to set theoffset for the sub-block may be determined based on information signaledfor each sub-block.

Whether to set the offset for the sub-block may be determined based on aposition of the sub-block, an index for identifying the sub-block in thecurrent block, etc. For example, based on a predetermined boundary ofthe current block, the offset may not set for the sub-block whichadjacent to the predetermined boundary, and the offset may be set forthe sub-block which is not adjacent to the predetermined boundary.

When assuming that the predetermined boundary is the top boundary of thecurrent block, under the intra prediction pattern corresponding to theindex ‘0’ or ‘1’, the offset may be not set for the sub-block which isadjacent to the top boundary of the current block, and the offset may beset for the sub-block which is not adjacent to the top boundary of thecurrent block.

When assuming that the predetermined boundary is the left boundary ofthe current block, under the intra prediction pattern corresponding tothe index ‘2’ or ‘3’, the offset may not be set for the sub-block whichis adjacent to the left boundary of the current block, and the offsetmay be set for the sub-block which is not adjacent to the left boundaryof the current block.

In FIG. 10, it is assumed that the offset is not set for one of thesub-blocks included in the current block but the offset is set foranother one. As another example, different values of the offset may beset for the sub-blocks included in the current block.

An example where different offset is set for each sub-block will bedescribed with reference to FIG. 11.

Referring to FIG. 11, when the index is ‘0’ or ‘1’, the offset ‘h’ maybe set for the upper sub-block of the current block, and the offset ‘f’may be set for the lower sub-block of the current block. Therefore, thesecond prediction sample (P(i,j)+h or P(i,j)−h) may be generated byadding or subtracting the offset ‘h’ to or from the first predictionsample in the upper sub-block, and the second prediction sample(P(i,j)+f or P(i,j)−f) may be generated by adding or subtracting theoffset ‘f’ to or from the first prediction sample.

Referring to FIG. 11, when the index is ‘2’ or ‘3’, the offset ‘h’ maybe set for the left sub-block of the current block, and the offset ‘f’may be set for the right sub-block of the current block. Therefore, thesecond prediction sample (P(i,j)+h or P(i,j)−h) may be generated byadding or subtracting the offset ‘h’ to or from the first predictionsample may in the left sub-block, and the second prediction sample(P(i,j)+f or P(i,j)−f) may be generated by adding or subtracting theoffset ‘f’ to or from the first prediction sample in the rightsub-block.

In FIGS. 10 and 11, the current block is partitioned into two sub-blockshaving the same size, but the number of sub-blocks and/or the size ofsub-blocks included in the current block is not limited to the examplesshown in FIGS. 10 and 11. The number of sub-blocks included in thecurrent block may be three or more, and the sub-blocks may havedifferent sizes.

When multiple intra prediction patterns are available, the availableintra prediction patterns may be grouped into multiple categories. Inthis case, the intra prediction pattern of the current block may beselected based on a first index for identifying a category and a secondindex identifying an intra prediction pattern in the category.

An example where the intra prediction pattern of the current block isdetermined based on the first index and the second index will bedescribed with reference to FIG. 12.

In the example shown in FIG. 12, 12 intra prediction patterns may beclassified into three categories each including four intra predictionpatterns. For example, intra prediction patterns corresponding toindexes 0 to 3 may be classified as a category 0, intra predictionpatterns corresponding to indexes 4 to 7 may be classified as a category1, and intra prediction patterns corresponding to indexes 8 to 11 may beclassified as a category 2.

The device for decoding a video may decode the first index from abitstream in order to specify the category including at least one intraprediction pattern. In the example shown in FIG. 12, the first index mayspecify one of the categories 0, 1, and 2.

When the category is specified based on the first index, the intraprediction pattern of the current block may be determined based on thesecond index decoded from a bitstream. When the category 1 is specifiedby the first index, the second index may specify one of the four intraprediction patterns (i.e., the index 4 to index 7) of the category 1.

In FIG. 12, it shows that categories include the same numbers of intraprediction patterns. But there is no need to the categories include thesame numbers of intra prediction patterns.

The number of available intra prediction patterns or of the number ofcategories may be determined in units of a sequence or a slice. Also, atleast one of the number of available intra prediction patterns and thenumber of categories may be signaled through a sequence header or aslice header.

As another example, the number of available intra prediction patternsand/or the number of categories may be determined based on a size of aprediction unit or a coding unit of the current block. For example, whenthe size of the current block (e.g., the coding unit of the currentblock) is equal to or greater than 64×64, the intra prediction patternof the current block may be selected from five intra prediction patternsshown in FIG. 13. In contrast, when the size of the current block (e.g.,the coding unit of the current block) is less than 64×64, the intraprediction pattern of the current block may be selected from intraprediction patterns shown in FIG. 10, 11, or 12.

In FIGS. 10 to 13, it is depicted that the sub-blocks included in eachintra prediction pattern have a rectangular shape. As another example,the intra prediction pattern where at least one of the sizes and shapesof the sub-blocks are different from each other may be used. Forexample, FIG. 14 is a view illustrating an example of an intraprediction pattern with different sizes and shapes of sub-blocks.

The offset for each sub-block (e.g., the offset h, f, g, or i of eachsub-block shown in FIGS. 10 to 14) may be decoded from a bitstream, ormay be derived from the neighboring sample adjacent to the currentblock.

As another example, the offset of the sub-block may be determinedconsidering the distance from a sample at a particular position in thecurrent block. For example, the offset may be determined in proportionto a value representing the distance between a sample at a predeterminedposition in the current block and a sample at a predetermined positionin the sub-block.

As another example, the offset of the sub-block may be determined addingor subtracting a value determined based on the distance between a sampleat a predetermined position in the current block and a sample at apredetermined position in the sub-block to or from a pre-set value.

As another example, the offset may be determined based on a ratio of avalue representing the size of the current block and a valuerepresenting the distance between a sample at a predetermined positionin the current block and a sample at a predetermined position in thesub-block.

Here, the sample at the predetermined position in the current block mayinclude a sample adjacent to the left boundary of the current block, asample positioned at the top boundary of the current block, a sampleadjacent to the top left corner of the current block, etc.

FIG. 15 is a view illustrating a method of performing prediction usingan intra block copy scheme according to an embodiment of the presentinvention.

Intra block copy (IBC) is a method where a current block ispredicted/reconstructed using a block (hereinafter, referred to as ‘areference block’) already reconstructed in the same picture as thecurrent block. If a picture contains a large number of letters, such asKorean alphabet, an alphabet, etc. and a letter which is contained inthe current block when the current block is reconstructed are containedin an already decoded block, intra block copy can enhance anencoding/decoding performance.

An intra block copy method may be classified as an intra predictionmethod or an inter prediction method. When the intra block copy methodis classified as the intra prediction method, an intra prediction modefor the intra block copy method may be defined. When the intra blockcopy method is classified as the inter prediction method, a bitstreammay include a flag indicating whether to apply the intra block copymethod to the current block. Alternatively, whether the current blockuses intra block copy may be confirmed through a reference picture indexof the current block. That is, when the reference picture index of thecurrent block indicates the current picture, inter prediction may beperformed on the current block using intra block copy. To this end, apre-reconstructed current picture may be added to a reference picturelist for the current block. The current picture may exist at a fixedposition in the reference picture list (e.g., a position with thereference picture index of 0 or the last position). Alternatively, thecurrent picture may have a variable position in the reference picturelist, and to this end, a reference picture index indicating a positionof the current picture may be signaled, separately.

In order to specify the reference block of the current block, a positiondifference between the current block and the reference block may bedefined as a motion vector (hereinafter, referred to as a block vector).

The block vector may be derived by a sum of a prediction block vectorand a differential block vector. The device for encoding a video maygenerate a prediction block vector through predictive coding, and mayencode the differential block vector indicating the difference betweenthe block vector and the prediction block vector. In this case, thedevice for decoding a video may derive the block vector of the currentblock by using the prediction block vector derived using pre-decodedinformation and the differential block vector decoded from a bitstream.

Here, the prediction block vector may be derived based on the blockvector of a neighboring block adjacent to the current block, the blockvector in an LCU of the current block, the block vector in an LCUrow/column of the current block, etc.

The device for encoding a video may encode the block vector withoutperforming predictive coding of the block vector. In this case, thedevice for decoding a video may obtain the block vector by decodingblock vector information signaled through a bitstream. The correctionprocess may be performed on the prediction/reconstructed samplegenerated through the intra block copy method. In this case, thecorrection method described with reference to FIGS. 6 to 14 may beequally/similarly applied, and thus detailed description thereof will beomitted.

FIG. 16 shows a range of reference samples for intra predictionaccording to an embodiment to which the present invention is applied.

Referring to FIG. 16, intra prediction may performed by using referencesamples P (−1, −1), P (−1, y) (0<=y<=2N−1) and P (x, −1) (0<=x<=2N−1)located at a boundary of a current block. At this time, filtering onreference samples is selectively performed based on at least one of anintra prediction mode (e.g., index, directionality, angle, etc. of theintra prediction mode) of the current block or a size of a transformblock related to the current block.

At least one of a plurality of intra filter candidates may be selectedto perform filtering on reference samples. Here, the plurality ofintra-filter candidates may differ from each other in at least one of afilter strength, a filter coefficient or a tap number (e.g., a number offilter coefficients, a filter length). A plurality of intra-filtercandidates may be defined in at least one of a sequence, a picture, aslice, or a block level. That is, a sequence, a picture, a slice, or ablock in which the current block is included may use the same pluralityof intra-filter candidates.

Hereinafter, for convenience of explanation, it is assumed that aplurality of intra-filter candidates includes a first intra-filter and asecond intra-filter. It is also assumed that the first intra-filter is a(1, 2, 1) 3-tap filter and the second intra-filter is a (2, 3, 6, 3, 2)5-tap filter.

When reference samples are filtered by applying a first intra-filter,the filtered reference samples may be derived as shown in Equation 7.P(−1,−1)=(P(−1,0)+2P(−1,−1)+P(0,−1)+2)>>2P(−1,y)=(P(−1,y+1)+2P(−1,y)+P(−1,y−1)+2)>>2P(x,−1)=(P(x+1,−1)+2P(x,−1)+P(x−1,−1)+2)>>2  [Equation 7]

When reference samples are filtered by applying the second intra-filter,the filtered reference samples may be derived as shown in the followingequation 8.P(−1,−1)=(2P(−2,0)+3P(−1,0)+6P(−1,−1)+3P(0,−1)+2P(0,−2)+8)>>4P(−1,y)=(2P(−1,y+2)+3P(−1,y+1)+6P(−1,y)+3P(−1,y−1)+2P(−1,y−2)+8)>>4P(x,−1)=(2P(x+2,−1)+3P(x+1,−1)+6P(x,−1)+3P(x−1,−1)+2P(x−2,−1)+8)>>4  [Equation8]

Based on a position of a reference sample, one of a plurality ofintra-filter candidates may be determined and used to perform filteringon the reference sample by using the determined one. For example, afirst intra-filter may be applied to a reference sample at a boundary ofa current block, and a second intra-filter may be applied to otherreference samples. Specifically, as shown in FIG. 17, filtering onreference samples P (−1, −1), P (−1, 0), P (−1, 1), . . . , P (−1, N−1)and P (0, −1), P (1, −1), . . . is performed by applying a firstintra-filter as shown in Equation 7, and filtering on the otherreference samples is performed by applying a second reference filter asshown in Equation 8.

It is possible to select one of a plurality of intra-filter candidatesbased on a transform type used for a current block, and performfiltering on reference samples using the selected one. Here, thetransform type may mean (1) a transform scheme such as DCT, DST or KLT,(2) a transform mode indicator such as a 2D transform, 1D transform ornon-transform or (3) the number of transforms such as a first transformand a second transform. Hereinafter, for convenience of description, itis assumed that the transform type means the transform scheme such asDCT, DST and KLT.

For example, if a current block is encoded using a DCT, filtering may beperformed using a first intra-filter, and if a current block is encodedusing a DST, filtering may be performed using a second intra-filter. Or,if a current block is encoded using DCT or DST, filtering may beperformed using a first intra-filter, and if the current block isencoded using a KLT, filtering may be performed using a secondintra-filter.

Filtering may be performed using a filter selected based on a transformtype of a current block and a position of a reference sample. Forexample, if a current block is encoded using the a DCT, filtering onreference samples P (−1, −1), P (−1, 0), P (−1, 1), . . . , P (−1, N−1)and P (0, −1), P (1, −1), . . . , P (N−1, −1) may be performed by usinga first intra-filter, and filtering on other reference samples may beperformed by using a second intra-filter. If a current block is encodedusing a DST, filtering on reference samples P (−1, −1), P (−1, 0), P(−1, 1), . . . , P (−1, N−1) and P (0, −1), P (1, −1), . . . , P (N−1,−1) may be performed by using a second intra-filter, and filtering onother reference samples may be performed by using a first intra-filter.

One of a plurality of intra-filter candidates may be selected based onwhether a transform type of a neighboring block including a referencesample is the same as a transform type of a current block, and thefiltering may be performed using the selected intra-filter candidate.For example, when a current block and a neighboring block use the sametransform type, filtering is performed using a first intra-filter, andwhen transform types of a current block and of a neighboring block aredifferent from each other, the second intra-filter may be used toperform filtering.

It is possible to select any one of a plurality of intra filtercandidates based on a transform type of a neighboring block and performfiltering on a reference sample using the selected one. That is, aspecific filter may be selected in consideration of a transform type ofa block in which a reference sample is included. For example, as shownin FIG. 18, if a block adjacent to left/lower left of a current block isa block encoded using a DCT, and a block adjacent to top/top right of acurrent block is a block encoded using a DST, filtering on referencesamples adjacent to left/lower left of a current block is performed byapplying a first intra filter and filtering on reference samplesadjacent to top/top right of a current block is performed by applying asecond intra filter.

In units of a predetermined region, a filter usable in the correspondingregion may be defined. Herein, the unit of the predetermined region maybe any one of a sequence, a picture, a slice, a block group (e.g., a rowof coding tree units) or a block (e.g., a coding tree unit) Or, anotherregion may be defined that shares one or more filters. A referencesample may be filtered by using a filter mapped to a region in which acurrent block is included.

For example, as shown in FIG. 19, it is possible to perform filtering onreference samples using different filters in CTU units. In this case,information indicating whether the same filter is used in a sequence ora picture, a type of filter used for each CTU, an index specifying afilter used in the corresponding CTU among an available intra filtercandidates may be signaled via a sequence parameter set (SPS) or apicture parameter set (PPS).

An intra-prediction mode may be effectively used when a current picturehas no continuity with a previous picture or when a current pictureincludes a texture having directionality. Particularly, when a currentpicture includes a texture having directionality, an intra-predictionmode of the current block is likely to have the identical or similardirection as an intra-prediction mode of a neighboring block.Accordingly, an intra-prediction mode of a current block may beencoded/decoded based on an intra-prediction mode of a neighboringblock. Specifically, an intra-prediction mode of a current block may beencoded/decoded based on a plurality of MPM (Most Probable mode)candidates generated based on an intra-prediction mode of a neighboringblock.

For example, in encoding process, when an intra-prediction mode of acurrent block is identical to any one of MPM candidates, theintra-prediction mode of the current block may be encoded by selectingan MPM candidate identical to the intra-prediction mode of the currentblock. Here, the encoder may encode index information (e.g., ‘mpm_idx’)indicating an MPM candidate identical to an intra-prediction mode of acurrent block among a plurality of MPM candidates. The index informationmay be signaled through a bitstream.

In decoding process, an intra-prediction mode of the current block maybe derived by information indicating whether any one of MPM candidatesis used as an intra-prediction mode of a current block, and by indexinformation (e.g., ‘mpm_idx’). Here, information indicating whether anyone of MPM candidates is used as the intra-prediction mode of a currentblock may be a 1-bit flag (e.g., ‘prev_intra_luma_pred_flag’), but isnot limited thereto.

For deriving an intra-prediction mode of a current block, a plurality ofMPM candidates may be used. A number of MPM candidates may be a fixednumber, and may be a variable number per a sequence, a slice, or atarget block to be decoded.

For example, a number of MPM candidates may be determined based oninformation signaled from a bitstream. A number of MPM candidates may bedetermined per sequence/slide by a syntax element, ‘num_mpm_idx,’signaled by unit of a sequence/slice.

A number of MPM candidates used for deriving an intra-prediction mode ofa current block may be a number from 3 to 6, but is not limited thereto.It is also possible to use more than 6 MPM candidates.

Hereinafter, assuming that extended intra-prediction modes illustratedin FIG. 4 are used, a method for deriving an intra-prediction mode of acurrent block using MPM candidates will be described in detail. Theembodiments described below may be applied not only when a larger numberof intra-prediction modes than those shown in FIG. 4 are used but alsowhen a smaller number of intra-prediction modes than those shown in FIG.4 are used.

FIG. 20 is a flowchart illustrating a method of deriving anintra-prediction mode of a current block, according to an embodiment ofthe present invention.

Referring to FIG. 20, firstly, MPM candidates are derived using anintra-prediction mode of a neighboring block adjacent to a current block(S2010). Here, a neighboring block adjacent to a current block may beblocks that have been decoded earlier than a current block, and mayinclude a top neighboring block adjacent to top of a current block, anda left neighboring block adjacent to left of a current block.

Here, a top neighboring block adjacent to a top side of a current blockmay include not only a neighboring block adjoining to a top boundary ofa current block but also a neighboring block adjacent to top corner of acurrent block (e.g., a left top corner of a current block or a right topcorner of a current block). In addition, when a top boundary of acurrent block is extended by a predetermined length, a block adjoiningto an extended top boundary or a block including coordinatescorresponding to an extended top boundary may be referred to as a topneighboring block of a current block.

A left neighboring block adjacent to a left side of a current block mayinclude not only a neighboring block adjoining to a left boundary of acurrent block but also a neighboring block adjacent to left corner of acurrent block (e.g., a left top corner of a current block or a leftbottom corner of a current block). In addition, when a left boundary ofa current block is extended by a predetermined length, a block adjoiningto an extended left boundary or a block including coordinatescorresponding to an extended left boundary may be referred to as a leftneighboring block of a current block.

MPM candidates of a current block may be generated using at least one ofan intra-prediction mode having a highest occurrence frequency amongintra-prediction modes of neighboring blocks adjacent to a currentblock, an intra-prediction mode having a largest value amongintra-prediction modes of neighboring blocks adjacent to a current blockor an intra-prediction mode having a smallest value amongintra-prediction modes of neighboring blocks adjacent to a currentblock.

For example, Table 3 shows an example in which MPM candidates aredetermined based on intra-prediction mode of neighboring blocks.

TABLE 3 MPM0 MPM1 MPM2 MPM3 MPM4 MPM5 (mpm_id (mpm_id (mpm_id (mpm_id(mpm_id (mpm_id Condition x = 0) x = 1) x = 2) x = 3) x = 4) x = 5) L =A L ≠ planar & L ≠ DC L Planar L + 1 L − 1 L + 2 DC Otherwise planar DCVer Hor 2 18 L ≠ A L ≠ planar & L = DC or A = DC L A Planar Max − 1Max + 1 Max + 2 R ≠ planar otherwise L A Planar DC Max + 1 Min − 1otherwise L + A < 2 L A Ver Hor 2 18 otherwise L A DC Max − 1 Max + 1Max + 2

In Table 3, L indicates an intra-prediction mode having a highestoccurrence frequency among intra-prediction modes of left neighboringblocks, and A indicates an intra-prediction mode having a highestoccurrence frequency among intra-prediction modes of top neighboringblocks. Max indicates a largest intra-prediction mode amongintra-prediction modes of left neighboring blocks and top neighboringblocks, and Min indicates a smallest intra-prediction mode of theintra-prediction modes of left neighboring blocks and top neighboringblocks. In Table 3, it is exemplified that 6 MPM candidates aregenerated. However, it is also possible to derive an intra-predictionmode of a current block by generating fewer or more MPM candidates.

In addition, instead of determining L or A based on occurrence frequencyof an intra-prediction mode, an intra-prediction mode of a neighboringblock at a specific position may be determined as L or A.

Referring to an example shown in Table 3, MPM candidates may bedetermined based on whether an intra-prediction modes L and A areidentical.

For example, when L and A are identical, and L and A are directionalmodes (i.e., neither Planar nor DC), MPM candidates of a current blockmay include at least one of L, an intra-prediction mode having adirection similar to L, a Planar mode and a DC mode. Here, anintra-prediction mode having a direction similar to an intra-predictionmode L means an intra-prediction mode in which a difference from theintra-prediction mode L is equal to or less than a threshold value. Forexample, an intra-prediction mode similar to an intra-prediction mode Lmay include L+1, L−1, L+2, or L−2, and so on. In Table 3, it isexemplified that 6 MPM candidates such as L, Planar, L+1, L−1, L+2 andDC are generated.

When L and A are identical, and L and A are non-directional modes (i.e.,Planar or DC), MPM candidates of a current block may include at leastone of a Planer mode, a DC mode and a pre-set directional mode. Here, apre-set directional mode may include Vertical and Horizontal, and mayinclude diagonal directional modes (e.g., 2, 34 or 66, etc.).Alternatively, a pre-set directional mode may have a direction differentfrom the examples above. In Table 3, it is exemplified that 6 MPMcandidates such as Planar, DC, Vertical (Ver), Horizontal (Hor), 2, and18 are generated.

When L and A are not identical, each of L and A may be set as an MPMcandidate. In addition, based on whether L and A are non-directionalmodes, at least one of Planar, DC, Max, an intra-prediction mode similarto Max, Min or an intra-prediction mode similar to Min may be generatedas an MPM candidate.

For example, when L and A are not identical, and L and A are not Planarmodes, but L or A is DC, at least one of L, A, Planar, Max or anintra-prediction mode similar to Max may be generated as an MPMcandidate. In Table 3, it is exemplified that 6 MPM candidates such asL, A, Planar, Max−1, Max+1 and Max+2 are generated.

When L and A are not identical, and L and A are neither Planar nor DC,at least one of L, A, non-directional mode, Max, Min, anintra-prediction mode similar to Max or an intra-prediction mode similarto Min may be generated as an MPM candidate. In Table 3, it isexemplified that 6 MPM candidates such as L, A, Planar, DC, Max+1, andMin−1 are generated.

When L and A are not identical but both of L and A are non-directionalmodes (i.e., one of L and A is Planar and the other is DC, that is,L+A<2), L, A and pre-set directional mode may be generated as an MPMcandidate. In Table 3, it is exemplified that 6 MPM candidates such asL, A, Vertical (Ver), Horizontal (Hor), 2 and 18 are generated.

When L and A are not identical, one of L and L is a Planar mode and theother is a directional mode, at least one of L, A, DC, Max or anintra-prediction mode similar to Max may be generated as an MPMcandidate. In Table 3, it is exemplified that 6 MPM candidates such asL, A, DC, Max−1, Max+1 and Max+2 are generated.

Hereinafter, referring to examples of FIG. 21, examples for generatingMPM candidates will be described in detail.

FIG. 21 is a diagram for explaining an example in which an MPM candidateis generated using an intra-prediction mode of a neighboring blockadjacent to a current block.

In FIG. 21, it is exemplified that a plurality of left neighboringblocks and a plurality of top neighboring blocks adjacent to the currentblock are present.

Referring to FIG. 21, an intra-prediction mode ‘L’ having a highestoccurrence frequency among intra-prediction modes of left neighboringblocks is 10. An intra-prediction mode ‘A’ having a highest occurrencefrequency among intra-prediction modes of top neighboring blocks is 26.In addition, a largest intra-prediction mode ‘Max’ for left neighboringblocks and top neighboring blocks is 26, and a smallest intra-predictionmode ‘Min’ for left neighboring blocks and top neighboring blocks is 0.Applying the above to the example shown in Table 3, 10, 26, Planar, 25,27 and 29 may be generated as MPM candidates for a current block.

A neighboring block used to generate MPM candidates of a current blockmay be limited to neighboring blocks included in a same coding tree unitas a current block. Here, a current block is adjacent to a left and topboundaries of a coding tree unit, and there may be no neighboring blockadjacent to left or top of the current block in the coding tree unit.Alternatively, when a neighboring block adjacent to left or top of acurrent block may not be a block encoded by intra-prediction mode, anintra-prediction mode of the neighboring block adjacent to the currentblock may not be used. In this case, MPM candidates of a current blockmay be generated based on predefined modes. Here, the predefined modesmay include at least one of DC, Planar, Vertical, or Horizontal.

For example, when there is no left neighboring blocks and topneighboring blocks adjacent to a current block, it is assumed that L isPlanar and A is DC, and MPM candidates of the current block aregenerated according to rules exemplified in Table 3.

An intra-prediction modes of one of top neighboring blocks and leftneighboring blocks may be used when generating MPM candidates, while theother may not be available when generating MPM candidates. Anunavailable intra-prediction mode may be replaced with an availableintra-prediction mode, thereby MPM candidates of a current block may begenerated.

Specifically, for generating MPM candidates of a current block, when anintra-prediction mode of a top neighboring block or an intra-predictionmode of a left neighboring block is not available, MPM candidates of thecurrent block may be generated based on a block closest to a neighboringblock of which intra-prediction mode is unavailable or anintra-prediction mode of a block most adjacent to a neighboring block ofwhich intra-prediction mode is unavailable.

For example, when an intra-prediction mode of a top neighboring block ofa current block is not available, an intra-prediction mode of a leftneighboring block closest (or most adjacent) to the top neighboringblock may be used instead of the intra-prediction mode of the topneighboring block. In addition, when an intra-prediction mode of a leftneighboring block of a current block is not available, anintra-prediction mode of a top neighboring block closest (or mostadjacent) to the left neighboring block may be used instead of theintra-prediction mode of the left neighboring block.

FIGS. 22A and 22B are a diagram showing an example of generating MPMcandidates when an intra-prediction mode of a top neighboring block or aleft neighboring block adjacent to a current block is not available.

When an intra-prediction mode of a top neighboring block is notavailable, an intra-prediction mode of a left neighboring block closestto the top neighboring block (i.e., a block closest to a top boundary ofa current block) among left neighboring blocks of a current block may beset as an intra-prediction mode A.

For example, referring to an example shown in FIG. 22A, anintra-prediction mode L having a highest occurrence frequency amongintra-prediction modes of left neighboring blocks is 10, and anintra-prediction mode A is 10 which is an intra-prediction mode of aleft neighboring block closest to a top neighboring block. When A and Lare determined, MPM candidates of a current block may be determinedaccording to examples of Table 3.

When an intra-prediction mode of a top neighboring block is notavailable but there are a plurality of left neighboring blocks adjacentto a current block, a decoder may sequentially obtain intra-predictionmodes of the left neighboring blocks. That is, a decoder may obtainintra-prediction modes of left neighboring blocks in an order ofadjacent to a top neighboring block (or top left neighboring block of acurrent block).

For example, in an example shown in FIG. 23A, a decoder may obtainsequentially, starting from a top left block of a current block,intra-prediction modes of neighboring blocks located below the top leftblock, and then, may obtain intra-prediction modes of neighboring blockslocated above the top left block. Accordingly, in an example shown inFIG. 23A, an intra-prediction mode of left neighboring blocks areobtained in an order of A0, A1, A2, A3, A4, B0, B1, among which afirstly available intra-prediction mode may be used instead of anintra-prediction mode (or intra-prediction mode A) of the topneighboring block.

When an intra-prediction mode of a left neighboring block is notavailable, an intra-prediction mode of a block closest to the leftneighboring block (i.e., a block closest to a left boundary of a currentblock) among top neighboring blocks of a current block may be set as anintra-prediction mode L.

For example, referring to an example shown in FIG. 22B, anintra-prediction mode A having a highest occurrence frequency amongintra-prediction modes of top neighboring blocks is 26, and anintra-prediction mode L is 26 which is an intra-prediction mode of aleft neighboring block closest to a left neighboring block. When L and Aare determined, MPM candidates of a current block may be determinedaccording to examples of Table 3.

When an intra-prediction mode of a left neighboring block is notavailable but there are a plurality of top neighboring blocks adjacentto a current block, a decoder may sequentially obtain intra-predictionmodes of the top neighboring blocks. That is, a decoder may obtainintra-prediction modes of top neighboring blocks in an order of adjacentto a left neighboring block (or top left neighboring block of a currentblock).

For example, in an example shown in FIG. 23B, a decoder may obtainsequentially, starting from a top left block of a current block,intra-prediction modes of neighboring blocks located at right side ofthe top left block, and then, may obtain intra-prediction modes ofneighboring blocks located at left side of the top left block.Accordingly, in an example shown in FIG. 23B, an intra-prediction modeof top neighboring blocks are obtained in an order of A0, A1, A2, A3,A4, B0, B1, among which a firstly available intra-prediction mode may beused instead of an intra-prediction mode (or intra-prediction mode L) ofthe left neighboring block).

For an additional example, when an intra-prediction mode of a topneighboring block is not available, an intra-prediction mode having ahighest occurrence frequency among intra-prediction modes of leftneighboring blocks may be used instead of an intra-prediction mode oftop neighboring block. For example, referring to an example shown inFIG. 23A, it is possible to use an intra-prediction mode having ahighest occurrence frequency among intra-prediction modes of blocks A0,A1, A2, A3, A4, B0 and B1 instead of an intra-prediction mode (orintra-prediction mode A) of the top neighboring block.

In addition, when an intra-prediction mode of a left neighboring blockis not available, an intra-prediction mode having a highest occurrencefrequency among intra-prediction modes of top neighboring blocks may beused instead of an intra-prediction mode of left neighboring block. Forexample, referring to an example shown in FIG. 23B, it is possible touse an intra-prediction mode having a highest occurrence frequency amongintra-prediction modes of blocks A0, A1, A2, A3, A4, B0 and B1 insteadof an intra-prediction mode (or intra-prediction mode L) of the leftneighboring block.

When MPM candidates of a current block are generated, it is determinedwhether there is an MPM candidate identical to an intra-prediction modeof a current block (S2020). Here, whether there is an MPM candidateidentical to a intra-prediction mode of a current block may be indicatedby information (e.g., ‘prev_intra_pred_flag’) signaled through abitstream. For example, prev_intra_pred_flag having value of 1 indicatesthat there is an MPM candidate identical to an intra-prediction mode ofa current block, and prev_intra_pred_flag having a value of 0 indicatesthat there is no MPM candidate identical to an intra-prediction mode ofthe a current block.

When it is determined that there is an MPM candidate identical to anintra-prediction mode of a current block, information specifying the MPMcandidate identical to the intra-prediction mode of the current blockmay be decoded (S2030), and based on the information, anintra-prediction mode of a current block may be determined.

On the other hand, when it is determined that there is no MPM candidateidentical to an intra-prediction mode of a current block, a remainingmode is decoded from a bitstream (S2040), and an intra-prediction modeof a current block may be determined based on the decoded remainingmode. A remaining mode is information being encoded to specify anintra-prediction mode of a current block, except for MPM candidatesamong intra-prediction modes. A decoder may determine anintra-prediction mode of a current block by comparing a remaining modeand MPM candidates.

When an extended intra-prediction mode is used, as a number ofintra-prediction modes increases, a number of bits used for encoding aremaining mode may also increase. For example, assuming that 67intra-prediction modes are used and 6 MPM candidates are used, aremaining mode should be long enough to represent 61 intra-predictionmodes. For example, when a remaining mode is assumed to be encoded as afixed length, the remaining mode should have a length of at least 6 bitsto represent the 61 intra-prediction modes.

Accordingly, in order to minimize signaling of a remaining predictionmode and more efficiently encode/decode an intra-prediction mode of acurrent block, an intra-prediction mode of a current block may bederived using a plurality of MPM candidate groups.

Hereinafter, a method for deriving an intra-prediction mode of a currentblock using a plurality of MPM candidate groups will be described indetail.

FIG. 24 shows an example of deriving an intra-prediction mode of acurrent block using 2 MPM candidate groups, FIG. 25 shows an example ofderiving an intra-prediction mode of a current block using 3 MPMcandidate groups.

Referring to FIG. 24, a decoder may determine based on a bitstreamwhether a MPM candidate identical to an intra-prediction mode of acurrent block is included in a first MPM candidate group (S2410). Here,whether or not an MPM candidate identical to an intra-prediction mode ofa current block is included in a first MPM candidate group may beindicated by first information (e.g., ‘prev_intra_pred_flag’) signaledthrough a bitstream.

When it is determined that an MPM candidate identical to anintra-prediction mode of a current block is included in a first MPMcandidate group, information (e.g., ‘mpm_idx’) specifying an MPMcandidate identical to a intra-prediction mode of a current block in afirst MPM candidate group (S2420) may be decoded, and anintra-prediction mode of a current block may be determined based on theinformation.

On the other hand, when it is determined that an MPM candidate identicalto an intra-prediction mode of a current block is not included in afirst MPM candidate group, it may be determined whether an MPM candidateidentical to an intra-prediction mode of a current block is included ina second MPM candidate group (S2430). Here, whether or not an MPMcandidate identical to an intra-prediction mode of a current block isincluded in a second MPM candidate group may be indicated by secondinformation (e.g., ‘2nd_prev_intra_pred_flag’) signaled through abitstream.

When it is determined that an MPM candidate identical to anintra-prediction mode of a current block is included in a second MPMcandidate group, information (e.g., ‘2nd_mpm_idx’) specifying an MPMcandidate identical to an intra-prediction mode of a current block in asecond MPM candidate group may be decoded (S2440), and anintra-prediction mode of a current block may be determined based on theinformation.

On the other hand, when it is determined that none of a first MPMcandidate group and a second MPM candidate group include an MPMcandidate identical to an intra-prediction mode of a current block, aremaining mode is decoded from a bitstream (S2450), and anintra-prediction mode of a current block may be determined based on thedecoded remaining mode.

Here, a first MPM candidate group may include an MPM candidate generatedbased on intra-prediction modes of neighboring blocks of a currentblock. For example, a first MPM candidate group may include 6 MPMcandidates generated through an example of Table 3.

A second MPM candidate group may include an MPM candidate not includedin a first MPM candidate group. For example, in Table 3, when L and Aare identical, and L and A are directional modes, a first MPM candidategroup may include MPM candidates corresponding to Planar, DC, Vertical,Horizontal, 2 and 18. Accordingly, a second MPM candidate group mayinclude MPM candidates other than Planar, DC, Vertical, Horizontal, 2and 18.

A second MPM candidate group may include a pre-set intra-prediction modeor an MPM candidate generated based on MPM candidates included in afirst MPM candidate group. For example, a second MPM candidate group mayinclude an MPM candidate having a direction similar to a directional MPMcandidate included in a first MPM candidate group.

For an additional example, one of a first MPM candidate group and asecond MPM candidate group is generated based on an intra-predictionmode of a left neighboring block of a current block, and the other isgenerated based on an intra-prediction mode of a top neighbor block of acurrent block.

A number of MPM candidates included in a second MPM candidate group maybe equal to or greater than a number of MPM candidates included in afirst MPM candidate group. For example, a first MPM candidate group mayinclude maximum of 6 MPM candidates, while a second MPM candidate groupmay include maximum of 16 MPM candidates.

When maximum of 6 MPM candidates are included in a first MPM candidategroup, information (e.g., ‘mpm_idx’) for identifying an MPM candidateincluded in a first MPM candidate group may be encoded as a fixed lengthof 3 bits. On the other hand, when a maximum of 16 MPM candidates areincluded in a second MPM candidate group, information (e.g.,‘2nd_mpm_idx’) for identifying an MPM candidate included in a second MPMcandidate group may be encoded as a fixed length of 4 bits. When 6 MPMcandidates are included in a first MPM candidate group and 16 MPMcandidates are included in a second MPM candidate group, the remainingmode is encoded as a fixed length of 6 bits to identify 45intra-prediction modes.

Referring to FIG. 25, a decoder may determine based on a bitstreamwhether an MPM candidate identical to an intra-prediction mode of acurrent block is included in a first MPM candidate group (S2510). Here,whether or not an MPM candidate identical to an intra-prediction mode ofa current block is included in a first MPM candidate group may beindicated by first information (e.g., ‘prev_intra_pred_flag’) signaledthrough a bitstream.

When it is determined that an MPM candidate identical to anintra-prediction mode of a current block is included in a first MPMcandidate group, information (e.g., ‘mpm_idx’) specifying an MPMcandidate identical to a intra-prediction mode of a current block in afirst MPM candidate group (S2520) may be decoded, and anintra-prediction mode of a current block may be determined based on theinformation.

On the other hand, when it is determined that an MPM candidate identicalto an intra-prediction mode of a current block is not included in afirst MPM candidate group, it may be determined whether an MPM candidateidentical to an intra-prediction mode of a current block is included ina second MPM candidate group (S2530). Here, whether or not an MPMcandidate identical to an intra-prediction mode of a current block isincluded in a second MPM candidate group may be indicated by secondinformation (e.g., ‘2nd_prev_intra_pred_flag’) signaled through abitstream.

When it is determined that an MPM candidate identical to anintra-prediction mode of a current block is included in a second MPMcandidate group, information (e.g., ‘2nd_mpm_idx’) specifying an MPMcandidate identical to an intra-prediction mode of a current block in asecond MPM candidate group may be decoded (S2540), and anintra-prediction mode of a current block may be determined based on theinformation.

On the other hand, when it is determined that an MPM candidate identicalto an intra-prediction mode of a current block is not included in asecond MPM candidate group, it may be determined whether an MPMcandidate identical to an intra-prediction mode of a current block isincluded in a third MPM candidate group (S2550). Here, whether or not anMPM candidate identical to an intra-prediction mode of a current blockis included in a third MPM candidate group may be indicated by secondinformation (e.g., ‘3rd_prev_intra_pred_flag’) signaled through abitstream.

When it is determined that an MPM candidate identical to anintra-prediction mode of a current block is included in a third MPMcandidate group, information (e.g., ‘3rd_mpm_idx’) specifying an MPMcandidate identical to an intra-prediction mode of a current block in athird MPM candidate group may be decoded (S2540), and anintra-prediction mode of a current block may be determined based on theinformation.

On the other hand, when it is determined that none of a first MPMcandidate group, a second MPM candidate group and a third MPM candidategroup include an MPM candidate identical to an intra-prediction mode ofa current block, a remaining mode is decoded from a bitstream (S2570),and an intra-prediction mode of a current block may be determined basedon the decoded remaining mode.

A third MPM candidate group may include an MPM candidate not included ineither a first MPM candidate group or a second MPM candidate group. MPMcandidates included in a third MPM candidate group may be generatedbased on the pre-set intra-prediction mode, MPM candidates included in afirst MPM candidate group or MPM candidates included in a second MPMcandidate group. For example, a third MPM candidate group may include anMPM candidate having a direction similar to a directional MPM candidateincluded in a first MPM candidate group or a first MPM candidate group.

A number of MPM candidates included in a third MPM candidate group maybe equal to or greater than a number of MPM candidates included in afirst MPM candidate group. In addition, a number of MPM candidatesincluded in a third MPM candidate group may be equal to or greater thana number of MPM candidates included in a second MPM candidate group. Forexample, a first MPM candidate group may include 6 MPM candidates, whilea second MPM candidate group and a third MPM candidate group may include16 MPM candidates.

When a third MPM candidate group includes 16 MPM candidates, information(e.g., ‘3rd_mpm_idx’) for identifying MPM candidate included in a thirdMPM candidate group may be encoded as a fixed length of 4 bits. When afirst MPM candidate group includes 6 MPM candidates, and each of asecond MPM candidate group and a third MPM candidate group includes 16MPM candidates, a remaining mode may be encoded as a fixed length of 5bits to identify 29 intra-prediction modes.

Referring to FIGS. 24 and 25, it is illustrated that an intra-predictionmode of a current block is obtained using 2 and 3 MPM candidate groups,respectively. A number of MPM candidate groups used to derive anintra-prediction mode of a current block is not limited to the examplesdescribed. In addition, a number of MPM candidates included in a firstto third MPM candidate groups is not limited to the examples described.

Whether or not to use a plurality of MPM candidate groups may bedetermined based on information signaled in units of a sequence, apicture, a slice, a unit to be encoded/decoded. Here, the informationmay be information indicating a maximum number of MPM candidates or flaginformation indicating whether to use a plurality of MPM candidategroups.

When an intra-prediction mode of a current block is determined,intra-prediction may be performed using a reference sample adjacent tothe current block. For example, prediction samples of a current blockmay be generated by averaging reference samples, or may be generated byduplicating reference samples in a specific direction considering adirectionality of an intra-prediction mode. As described above in anexample referring to FIG. 16, P(−1, −1), P(−1, y) (0<=y<=2N−1), P(x, −1)(0<=x<=2N−1) which are located at a boundary of a current block may beused as reference samples.

When it is determined that a reference sample is not available, areference sample that is not available may be replaced with a referencesample that is available. Here, a reference sample may be determined asunavailable in case where a position of a reference sample is outside apicture, a reference sample is present in a slice different from acurrent block, or a reference sample is included in a block encoded byan inter-prediction. Here, whether or not a reference sample included ina block encoded by an inter-prediction is unavailable may be determinedbased on information indicating whether to use a reference sampleincluded in a block encoded by an inter-prediction when performingintra-prediction of a current block. Here, the information may be a1-bit flag (e.g., ‘constrained_intra_prediction_flag’), but is notlimited thereto. For example, when a value of‘constrained_intra_prediction_flag’ is 1, a reference sample included ina block encoded by an inter-prediction may be determined to beunavailable.

In the example shown in FIG. 16, when it is determined that a referencesample located at left lowermost (e.g., P(−1, 2N−1)) is not available,the reference sample located at left lowermost may be replaced with afirst available reference sample as it is scanned in a predeterminedorder. Here, the scanning order may be sequentially performed from areference sample adjacent to the left lowermost reference sample. Forexample, in the example shown in FIG. 16, when P(−1, 2N−1) is notavailable, scanning may be performed in an order of P(−1, −2N−2) toP(−1, −1), P(−1) to P(2N−1, −1). P(−1, 2N−1) may be replaced with afirst available reference sample that is found as a result of the scan.

When a left reference sample except for a reference sample located atleft lowermost is unavailable, the left reference sample may be replacedwith a reference sample adjacent to a bottom of the left referencesample. For example, an unavailable reference sample P(−1, y) betweenP(−1, 2N−1) and P(−1, −1) may be replaced with a reference sample P(−1,y+1).

When a top reference sample is unavailable, the top reference sample maybe replaced with a reference sample adjacent to a left of the topreference sample. For example, an unavailable reference sample P(x, −1)between P(0, −1) and P(2N−1, −1) may be replaced with a reference sampleP(x−1, −1).

A reference sample set adjacent to a current block may be referred to asa ‘reference line’ (or ‘intra-reference line’ or ‘reference sampleline’). For example, in the example shown in FIG. 16, a ‘reference line’a reference sample set including P(−1, 2N−1) to P(−1, 1), P(0, −1) toP(2N−2, −1). An intra-prediction of a current block may be performedbased on reference samples included in a reference line. Anintra-prediction of a current block may be performed, using referencesamples included in a reference line, based on an intra-prediction modeof a current block, For example, when an intra-prediction mode of acurrent block is a DC mode, a prediction signal may be generated usingan average and weighted prediction of reference samples included in thereference line. For example, when an intra-prediction mode of a currentblock is a DC mode, prediction samples of the current block may beobtained according to Equation 9.P(0,0)=(P(−1,0)+P(0,−1)+2*dcVal)>>2P(x,0)=(P(x,−1)+3*dcVal)>>2P(0,y)=(P(−1,y)+3*dcVal)>>2  [Equation 9]

In Equation 9, dcVal may be generated based on an average value ofsamples except for P(−1, −1) among reference samples included in areference line.

A planar mode provides effective prediction efficiency in a smooth areahaving no strong edges, and is effective in improving discontinuity ofblock boundary or image quality deterioration of a block boundary. Whenan intra-prediction mode of a current block is a planar mode, ahorizontal direction provisional prediction sample of the current blockmay be obtained using a reference sample adjacent to a top right cornerof the current block and a reference sample having y coordinateidentical to the horizontal direction provisional prediction sample, anda vertical direction provisional prediction sample of the current blockmay be obtained using a reference sample adjacent to a bottom leftcorner of the current block and a reference sample having x coordinateidentical to the vertical direction provisional prediction sample. Forexample, a horizontal direction provisional prediction sample and avertical direction provisional prediction sample of a current block maybe obtained by according to Equation 10.P _(h)(x,y)=(N−1−x)*P(−1,y)+(x+1)*P(N,−1)Pv(x,y)=(N−1−y)*P(x,−1)+(y+1)*P(−1,N)  [Equation 10]

A prediction sample of a current block may be generated by summing ahorizontal direction provisional prediction sample and a verticaldirection provisional prediction sample, and then shifting the summationresult by a value determined according to a size of a current block. Forexample, a prediction sample of a current block may be obtainedaccording to Equation 11.P(x,y)=(P _(h)(x,y)+Pv(x,y)+N)>>(log 2(N)+1)  [Equation 11]

An intra-prediction of a current block may be performed using aplurality of reference lines. Assuming that a current block has a W×Hsize, M-th reference line may include p(−M, −M), reference sampleslocated in a row identical to p(−M, −M) (e.g., reference samples fromp(M+1, −M) to p(W+H+2(M−1), −M) or reference samples from p(−M+1, −M) top(2W+2(M−1), −M) and reference samples located in a column identical top(−M, −M) (e.g., reference samples from p(−M, −M+1) to p(−M, W+H+2(M−1))or reference samples from p(−M, −M+1) to p(−M, 2H+2(M−1))).

For example, FIG. 26 exemplifies a plurality of reference sample lines.As in the example shown in FIG. 26, when a first reference line adjacentto a boundary of a current block is referred to as a ‘reference line 0’,M-th reference line may be set adjacent to (M−1)-th reference line.

An intra-prediction of a current block may be performed by at least oneof a plurality of reference lines. A method of performingintra-prediction using a plurality of reference lines as described abovemay be referred to as an ‘intra-prediction method using an extendedreference sample’ or an ‘extended intra-prediction method.’

Whether or not performing intra-prediction using an extended referenceline may be determined based on information signaled through abitstream. Here, the information may be a 1-bit flag, but is not limitedthereto. Information on whether performing intra-prediction using anextended reference line may be signaled in units of a coding tree unit,an encoding unit or a prediction unit, or may be signaled in units of apicture or a slice.

Whether or not performing intra-prediction using an extended referenceline may be determined based on at least one of a size, shape, depth orintra-prediction mode of a current block.

When it is determined to perform intra-prediction using an extendedreference line, a decoder may determine a number of reference lines.Here, a number of reference lines may have a fixed value, and may beadaptively determined according to a size, shape or intra-predictionmode of a current block. For example, when an intra-prediction mode of acurrent block is a non-directional mode, intra-prediction of the currentblock is performed using one reference line. When an intra-predictionmode of a current block is a directional mode, intra-prediction of thecurrent block may be performed using a plurality of reference lines.

For an additional example, a number of reference lines may be determinedby information that is signaled in units of a sequence, a picture, aslice or a unit to be decoded. For example, a syntax element‘max_intra_line_idx’ indicating a number of reference lines available ina sequence or a slice may be signaled through a sequence header or aslice header.

Hereinafter, a method of performing intra-prediction using an extendedreference line will be described in detail.

FIG. 27 is a flowchart illustrating a method of performingintra-prediction using an extended reference line according to thepresent invention.

First, a decoder may generate a plurality of reference lines (S2710).Reference samples included in each reference line may be generated basedon reconstructed samples included in blocks decoded earlier than acurrent block. When an unavailable reference sample is included in areference line, the unavailable reference sample may be replaced with avalue of an available reference sample in the same reference line or anavailable reference sample in another reference line.

When an intra-prediction mode of a current block is a directional mode,a decoder may generate a reference line considering a directionality ofthe intra-prediction mode. Considering a directionality of anintra-prediction mode, a larger number of reference samples may beincluded in M-th reference line than in (M−1)-th reference line. Thatis, a reference line away from a current block may include a largernumber of reference samples than a reference line near the currentblock.

Here, a number of reference samples further included in M-th referenceline than in (M−1)-th reference line may be variably determinedaccording to a size of a current block.

For example, when a current block has a 4×4 size, M-th reference linemay further include four (specifically, 2 in horizontal direction and 2in vertical direction) reference samples than (M−1)-th reference line.In addition, when a current block has a size of 8×8, M-th reference linemay further include eight (specifically, 4 in horizontal direction and 4in vertical direction) reference samples than (M−1)-th reference line.

Referring to FIG. 26, as a size of a current block size is 4×4, it isexemplified that a first reference sample includes a total of 9reference samples and a second reference sample includes a total of 13(=9+2×2) reference samples.

When a current block is non-square, a number of reference samplesincluded in a reference line may be determined according to a horizontaland vertical lengths of a current block.

For an example, FIG. 28 is a diagram exemplifying a plurality ofreference lines for a non-square block. Describing with comparing FIGS.26 and 28, as a width of a current block decreases to ½, a number of topreference samples except for a top left reference sample included in areference line 0 is reduced from 8 to 4.

That is, considering FIGS. 26 and 28 together, when assuming that acurrent block has a W×H size, M-th reference line may include a total of2{(W+H)+2(M−1)}+1 reference samples including W+H+2(M−1) top referencesamples (or 2W+2(M−1) top reference samples) (i.e., horizontal directionreference samples), W+H+2(M−1) left reference samples (or 2H+2(M−1) leftreference samples) (i.e., vertical direction reference samples) and topleft reference sample.

A decoder may decode, based on a bitstream, index information specifyingone of a plurality of reference lines (S2820). For example, when 4reference lines are available as in the example shown in FIG. 26, indexinformation may specify any one of the 4 reference lines.

A reference line for performing intra-prediction for a current block maybe adaptively specified based on a size of a current block, a type of acurrent block, an intra-prediction mode of a current block, indexinformation in a neighboring block or a difference between anintra-prediction mode of a current block and a predeterminedintra-prediction mode, and the like.

When any one of a plurality of reference lines is specified, a decodermay perform intra-prediction for a current block using the specifiedreference line (S2730). When an intra-prediction mode of a current blockis a directional mode, a prediction sample of the current block may beobtained by a reference sample specified by the directional mode. Whenan intra-prediction mode of a current block points between referencesamples, a prediction sample of the current block may be generated basedon a weighted sum (weighted prediction) of a first reference sample anda second reference sample specified by the intra-prediction mode.

In the example described above, it is exemplified that index informationspecifying one of the plurality of reference lines is decoded aftergenerating a plurality of reference lines. It is also possible to obtainonly a reference line specified by index information among a pluralityof reference lines after decoding the index information specifying oneof the plurality of reference lines.

In the embodiment described above, it is described that intra-predictionfor a current block is performed using any one reference line specifiedby index information among a plurality of reference lines. It is alsopossible that intra-prediction for a current block may be performedusing two or more reference lines among a plurality of reference lines.Whether or not to use two or more reference lines in performingintra-prediction for a current block may be determined based oninformation signaled from a bitstream, a size of a current block, a typeof a current block, an intra-prediction mode of a current block, whetheran intra-prediction mode of a current block is a non-directional or adifference between an intra-prediction mode of a current block and apredetermined intra-prediction mode, and the like.

The two or more reference lines may be specified by a plurality of indexinformation signaled from a bitstream. For example, when two referencelines are set to be used, any one of the two reference lines may bespecified by first index information, and the other may be specified bysecond index information.

For an additional example, intra-prediction for a current block may beset to use two or more spatially contiguous reference lines. In thiscase, when any one of the two or more reference lines is specified byindex information signaled from a bitstream, the remaining referenceline may be specified in consideration of position with respect to thespecified reference line. For example, when it is set to use tworeference lines, and index information indicates ‘reference line 0,’then intra-prediction of a current block may be performed based onreference line 0 and reference line 1 neighboring the reference line 0.

When it is set to use a plurality of reference lines, intra-predictionof a current block may be performed based on an average value, a maximumvalue, a minimum value or a weighted sum of reference samples includedin the plurality of reference lines.

For example, assuming that an intra-prediction mode of a current blockis a directional mode (i.e., an Angular mode), a predicted sample of thecurrent block may be generated based on a first reference sample and asecond reference sample neighboring the first reference sample. Here, afirst reference sample may be included in a first reference line among aplurality of reference lines, and a second reference sample may beincluded in a second reference line among the plurality of referencelines. A first reference line and a second reference line may bepositioned neighboring each other, but it is not limited thereto. Afirst reference sample and a second reference sample may be specifiedaccording to a directionality of an intra-prediction mode of a currentblock. A prediction sample of a current block may be generated inconsideration of a weighted sum of a first reference sample and a secondreference sample, or may be generated based on an average value of afirst reference sample and a second reference sample.

Intra-prediction of a current block may be performed by performing afirst intra-prediction based on a part of a plurality of reference linesand performing a second intra-prediction based on the remainingreference lines. Here, an intra-prediction mode used in a firstintra-prediction and an intra-prediction mode used in a secondintra-prediction may be the same or different. A prediction sample of acurrent block may be generated based on a first prediction samplegenerated by performing a first intra-prediction and a second predictionsample generated by performing a second intra-prediction.

Above embodiments have been described mainly on decoding process,encoding process may be performed in the same order as described or inreverse order.

Although the above-described embodiments have been described on thebasis of a series of steps or flowcharts, they do not limit thetime-series order of the invention, and may be performed simultaneouslyor in different orders as necessary. Further, each of the components(for example, units, modules, etc.) constituting the block diagram inthe above-described embodiments may be implemented by a hardware deviceor software, and a plurality of components. Or a plurality of componentsmay be combined and implemented by a single hardware device or software.The above-described embodiments may be implemented in the form ofprogram instructions that may be executed through various computercomponents and recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include one of or combination ofprogram commands, data files, data structures, and the like. Examples ofcomputer-readable media include magnetic media such as hard disks,floppy disks and magnetic tape, optical recording media such as CD-ROMsand DVDs, magneto-optical media such as floptical disks, media, andhardware devices specifically configured to store and execute programinstructions such as ROM, RAM, flash memory, and the like. The hardwaredevice may be configured to operate as one or more software modules forperforming the process according to the present invention, and viceversa.

INDUSTRIAL APPLICABILITY

The present invention may be applied to electronic devices which is ableto encode/decode a video.

The invention claimed is:
 1. A method for decoding a video, the methodcomprising: determining a reference sample line of a current block byselecting one among a neighboring reference sample line adjacent to thecurrent block and non-neighboring reference sample lines not adjacent tothe current block; determining an intra-prediction mode of the currentblock; and performing intra-prediction for the current block based onthe intra-prediction mode and reference samples included in thereference sample line of the current block, wherein determining theintra-prediction mode of the current block comprises: decoding a firstflag and a second flag from a bitstream, each of the first flag and thesecond flag having 1-bit length, determining, based on the first flag,whether the intra-prediction mode of the current block is identical to afirst candidate intra-prediction mode in a first candidate list, whenthe first flag indicates that the intra-prediction mode of the currentblock is not identical to the first candidate intra-prediction modewhile the second flag indicates that the intra-prediction mode of thecurrent block is identical to one of the candidate intra predictionmodes, decoding an index information specifying one of candidateintra-prediction modes included in a second candidate list, anddetermining the intra-prediction mode of the current block based on acandidate intra-prediction mode specified by the index information,wherein when it is determined that none of the candidateintra-prediction modes is identical to the intra-prediction mode of thecurrent block, determining the intra-prediction mode of the currentblock comprises: decoding a remaining mode, and deriving theintra-prediction mode of the current block by comparing the decodedremaining mode with a candidate intra-prediction mode, wherein a planarmode is included in the first candidate list as the first candidateintra-prediction mode, and the second candidate list is constitutedwithout comprising the planar mode, wherein at least one of thecandidate intra-prediction modes is derived based on a secondintra-prediction mode of a neighboring block adjacent to the currentblock, wherein when a second intra prediction scheme is applied to theneighboring block while a first intra prediction scheme is applied to acurrent block, the at least one of the candidate intra-prediction modesis derived by using a predetermined intra-prediction mode, wherein thenumber of intra-prediction modes defined under the first intraprediction scheme is different from the number of intra-prediction modesdefined under the second intra prediction scheme, wherein, under anon-directional intra prediction mode of a Planar or a DC, the intraprediction for the current block is performed by using only theneighboring reference sample line but cannot be performed by using anyone among the non-neighboring reference sample lines, and wherein underanother intra prediction mode different from the non-directional intraprediction mode, the intra prediction for the current block is performedby using any one among the neighboring reference sample line and thenon-neighboring reference sample lines.
 2. The method according to claim1, wherein the second candidate list comprises a candidateintra-prediction mode derived by adding or subtracting an offset from amaximum among intra-prediction modes of a left neighboring block and atop neighboring block.
 3. The method according to claim 1, wherein whenthe decoded remaining mode is equal to or greater than the candidateintra-prediction mode, the intra-prediction mode of the current block isderived by adding 1 to the decoded remaining mode.
 4. The methodaccording to claim 3, wherein the remaining mode is encoded as a fixedlength.
 5. A method for encoding a video, the method comprising:determining a reference sample line of a current block by selecting oneamong a neighboring reference sample line adjacent to the current blockand non-neighboring reference sample lines not adjacent to the currentblock; determining an intra-prediction mode of the current block; andperforming intra-prediction for the current block based on theintra-prediction mode and reference samples included in the referencesample line of the current block, wherein determining theintra-prediction mode of the current block comprises: encoding a firstflag and a second flag into a bitstream, each of the first flag and thesecond flag having 1-bit length, the first flag being encoded based onwhether the intra-prediction mode of the current block is identical to afirst candidate intra-prediction mode in a first candidate list, andwhen it is determined that the intra-prediction mode of the currentblock is not equal to the first candidate intra-prediction mode in thefirst candidate list while it is determined that one of candidateintra-prediction modes is identical to the intra-prediction mode of thecurrent block, encoding index information specifying one identical tothe intra-prediction mode of the current block among candidateintra-prediction modes in a second candidate list; and wherein when itis determined that none of the candidate intra-prediction modes isidentical to the intra-prediction mode of the current block, determiningthe intra-prediction mode of the current block comprises: remappingremaining modes by comparing the intra-prediction mode of the currentblock with a candidate intra-prediction mode, and encoding a remainingmode corresponding to the intra-prediction mode of the current block,wherein a planar mode is included in the first candidate list as thefirst candidate intra-prediction mode, and the second candidate list isconstituted without comprising the planar mode, wherein at least one ofthe candidate intra-prediction modes is derived based on a secondintra-prediction mode of a neighboring block adjacent to the currentblock, wherein when a second intra prediction scheme is applied to theneighboring block while a first intra prediction scheme is applied to acurrent block, the at least one of the candidate intra-prediction modesis derived by using a predetermined intra-prediction mode, wherein thenumber of intra-prediction modes defined under the first intraprediction scheme is different from the number of intra-prediction modesdefined under the second intra prediction scheme, wherein, under anon-directional intra prediction mode of a Planar or a DC, the intraprediction for the current block is performed by using only theneighboring reference sample line but cannot be performed by using anyone among the non-neighboring reference sample lines, and wherein underanother intra prediction mode different from the non-directional intraprediction mode, the intra prediction for the current block is performedby using any one among the neighboring reference sample line and thenon-neighboring reference sample lines.
 6. The method according to claim5, wherein the second candidate list comprises a candidateintra-prediction mode derived by adding or subtracting an offset from amaximum among intra-prediction modes of a left neighboring block and atop neighboring block.
 7. An apparatus for decoding a video, theapparatus comprising: one or more processors configured to: a decodingunit to decode a first flag and a second flag from a bitstream, each ofthe first flag and the second flag having 1-bit length; and anintra-prediction unit to determine a reference sample line of a currentblock by selecting one among a neighboring reference sample lineadjacent to the current block and non-neighboring reference sample linesnot adjacent to the current block, to determine the intra-predictionmode of the current block, and to perform intra-prediction for thecurrent block based on the intra-prediction mode and reference samplesincluded in the reference sample line of the current block, wherein theintra-prediction unit is further configured to: determine, based on thefirst flag, whether the intra-prediction mode of the current block isidentical to a first candidate intra-prediction mode in a firstcandidate list, and when the first flag indicates that theintra-prediction mode of the current block is not identical to the firstcandidate intra-prediction mode while the second flag indicates that theintra-prediction mode of the current block is identical to one of thecandidate intra prediction modes, determining the intra-prediction modeof the current block based on index information specifying one ofcandidate intra-prediction modes included in a second candidate list,wherein when it is determined that none of the candidateintra-prediction modes is identical to the intra-prediction mode of thecurrent block, the intra-prediction unit is further configured todetermine the intra-prediction mode of the current block by comparing aremaining mode with a candidate intra-prediction mode, the remainingmode being parsed from a bitstream, wherein a planar mode is included inthe first candidate list as the first candidate intra-prediction mode,and the second candidate list is constituted without comprising acandidate intra-prediction mode of the planar mode, wherein at least oneof the candidate intra-prediction modes is derived based on a secondintra-prediction mode of a neighboring block adjacent to the currentblock, wherein when a second intra prediction scheme is applied to theneighboring block while a first intra prediction scheme is applied to acurrent block, the at least one of the candidate intra-prediction modesis derived by using a predetermined intra-prediction mode, wherein thenumber of intra-prediction modes defined under the first intraprediction scheme is different from the number of intra-prediction modesdefined under the second intra prediction scheme, wherein, under anon-directional intra prediction mode of a Planar or a DC, the intraprediction for the current block is performed by using only theneighboring reference sample line but cannot be performed by using anyone among the non-neighboring reference sample lines, and wherein underanother intra prediction mode different from the non-directional intraprediction mode, the intra prediction for the current block is performedby using any one among the neighboring reference sample line and thenon-neighboring reference sample lines is utilizable as the referencesample line of the current block.
 8. A non-transitory computer-readablemedium for storing data associated with a video signal, comprising: adata stream stored in the non-transitory computer-readable medium, thedata stream being decoded by a decoding method which comprising:determining a reference sample line of a current block by selecting oneamong a neighboring reference sample line adjacent to the current blockand non-neighboring reference sample lines not adjacent to the currentblock; determining an intra-prediction mode of the current block; andperforming intra-prediction for the current block based on theintra-prediction mode and reference samples included in the referencesample line of the current block, wherein determining theintra-prediction mode of the current block comprises: decoding a firstflag and a second flag from a bitstream, each of the first flag and thesecond flag having 1-bit length, determining, based on the first flag,whether the intra-prediction mode of the current block is identical to afirst candidate intra-prediction mode in a first candidate list, whenthe first flag indicates that the intra-prediction mode of the currentblock is not identical to the first candidate intra-prediction modewhile the second flag indicates that the intra-prediction mode of thecurrent block is identical to one of candidate intra prediction modes,decoding an index information specifying one of candidateintra-prediction modes included in a second candidate list, anddetermining the intra-prediction mode of the current block based on acandidate intra-prediction mode specified by the index information,wherein when it is determined that none of the candidateintra-prediction modes is identical to the intra-prediction mode of thecurrent block, determining the intra-prediction mode of the currentblock comprises: decoding a remaining mode, and deriving theintra-prediction mode of the current block by comparing the decodedremaining mode with a candidate intra-prediction mode, wherein a planarmode is included in the first candidate list as the first candidateintra-prediction mode, and the second candidate list is constitutedwithout comprising the planar mode, wherein at least one of thecandidate intra-prediction modes is derived based on a secondintra-prediction mode of a neighboring block adjacent to the currentblock, wherein when a second intra prediction scheme is applied to theneighboring block while a first intra prediction scheme is applied to acurrent block, the at least one of the candidate intra-prediction modesis derived by using a predetermined intra-prediction mode, wherein thenumber of intra-prediction modes defined under the first intraprediction scheme is different from the number of intra-prediction modesdefined under the second intra prediction scheme, wherein, under anon-directional intra prediction mode of a Planar or a DC, the intraprediction for the current block is performed by using only theneighboring reference sample line but cannot be performed by using anyone among the non-neighboring reference sample lines, and wherein underanother intra prediction mode different from the non-directional intraprediction mode, the intra prediction for the current block is performedby using any one among the neighboring reference sample line and thenon-neighboring reference sample lines is utilizable as the referencesample line of the current block.
 9. The method of claim 1, wherein,under a case that the current block is partitioned into a plurality ofpartitions, the intra prediction for the current block is performed byusing only the neighboring reference sample line but cannot be performedby using any one among the non-neighboring reference sample lines, andwherein, under a case that the current block is not partitioned into theplurality of partitions, the intra prediction for the current block isperformed by using any one among the neighboring reference sample lineand the non-neighboring reference sample lines.
 10. The method of claim9, wherein when it is determined to partition the current block into theplurality of partitions, the number of partitions is adaptivelydetermined as two or four based on a size of the current block.
 11. Themethod of claim 1, wherein the number of candidate reference samplelines is determined based on information signaled via a sequenceparameter set.
 12. The method of claim 1, wherein the method furthercomprises determining whether to modify prediction samples included inthe current block generated by the intra prediction, and wherein when itis determined to modify the prediction samples, whether each of theprediction samples is modified or not is determined based on a positionthereof.
 13. The method of claim 12, wherein modifying the predictionsample is performed by using an offset derived based on a neighboringsample, and wherein a weight applied to the offset is proportional orinverse-proportional to a distance from a boundary of the current blockto the prediction sample.